Fix a problem with mod_perl on Windows using VS2010+.

The problem is caused by the following two commits, which sought to deal with
new Exxx values (with values >= 100) in errno.h which Microsoft added in VS2010:

http://perl5.git.perl.org/perl.git/commit/b59e75b34cef3fedd214c9b6ee744146cf8b3308
http://perl5.git.perl.org/perl.git/commit/912c63ed00375338703043928cac3c740d00cc9d

The former commit was mostly a patch to Errno and POSIX, together with some
other cleaning up in win32/win32.h and win32/include/sys/socket.h; the latter
commit was a fixup to win32/include/sys/socket.h which restored (more
aggressively) the ENOTSOCK->WSAENOTSOCK redefinition and added similar
redefinitions for ECONNABORTED, ECONNRESET and EAFNOSUPPORT.

It is the latter commit which causes this little program to output
ECONNABORTED=10053 rather than ECONNABORTED=106 as it ought to do in VC10 (and
higher) builds of perl:

#include <windows.h>
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
void main(void) {
  printf("ECONNABORTED=%d\n", ECONNABORTED);
}

That change is now causing problems with mod_perl, in which the (Perl level)
APR::Status::is_ECONNABORTED() and the (C level) APR_STATUS_IS_ECONNABORTED()
which it calls are failing to recognize an aborted connection (indicated by
error code ECONNABORTED set by Apache httpd.exe).

The APR_STATUS_IS_ECONNABORTED() macro is picked up by mod_perl from APR's
apr_errno.h:

#define APR_STATUS_IS_ECONNABORTED(s)   ((s) == APR_ECONNABORTED \
                || (s) == APR_OS_START_SYSERR + WSAECONNABORTED)

where

#ifdef ECONNABORTED
#define APR_ECONNABORTED ECONNABORTED
#else
#define APR_ECONNABORTED   (APR_OS_START_CANONERR + 18)
#endif

When this is compiled into httpd.exe ECONNABORTED is 106 (from errno.h) and
APR_STATUS_IS_ECONNABORTED(s) amounts to

#define APR_STATUS_IS_ECONNABORTED(s) ((s) == 106 || (s) == 730053)

but when compiled into APR/Status.dll ECONNABORTED is 10053 (redefined to
WSAECONNABORTED as above) so APR_STATUS_IS_ECONNABORTED(s) then amounts to

#define APR_STATUS_IS_ECONNABORTED(s) ((s) == 10053 || (s) == 730053)

which doesn't pick up an error code of 106 coming from httpd.exe.

This could be worked around in mod_perl by redefining ECONNABORTED and the other
three back to their original errno.h values to match what httpd.exe is using,
but that might just cause problems in the other direction, with those values no
longer matching the values which perl.exe is using.

Moreoever, this problem could affect other XS interfaces (not just mod_perl), so
it really needs to be fixed in perl.

This commit implements the alternative solution mentioned the commit message for
the first commit cited above. (As noted in that previous commit message, this
solution equally has potential problems, missing out on the advantages of the
original solution implemented, namely, better backwards compatibility with other
perl code having hard-coded numeric error codes, but this will be unavoidable if
we want to get to a sane state.)

Note that changing the $! values is an incompatible change, so should probably
not be done for 5.18.x. That's unfortunate for mod_perl (and anything else
similarly affected), but that will just have to either live with the breakage
(which was introduced in 5.14.0) until 5.20.0 or else try the workaround
mentioned above for 5.14.x, 5.16.x and 5.18.x.

The main change is to use a new function throughout win32/win32sck.c to convert
WSAGetLastError() values into errno.h constants before assigning to errno. Every
possible WSAExxx value (as documented by MSDN) is mapped to an Exxx value,
although it is still possible for other WSAxxx values to get assigned to errno
unchanged (but that has always been the case, and any non-existent Exxx values
get mapped back to WSAExxx values anyway...).

We must then ensure that all of those Exxx values are defined, which is now done
(in the new file, win32/include/sys/errno2.h) in a friendlier manner, being
careful not to redefine any (specifically the new ones in VC++ 2010 and above)
which already exist. The rest are defined as the WSAExxx values, as mentioned
above.

Finally, we need the Errno module to know about these values, which is done by
having it include that same new header file to ensure that it gets the same
definitions, rather than having to play its own games. The new header is also
used in POSIX, which similarly wants definitions of as many as possible of its
hard-coded list of Exxx values.
Fix IO::Socket::connect() in the light of $! changes

We now assign Exxx values rather than WSAExxx values to $! in the event of a
sockets error if there is an Exxx value corresponding to the WSAExxx error.
This is indeed the case for several Exxx values < 100, amongst them EINVAL (22),
which corresponds to WSAEINVAL (10022), which IO::Socket::connect() has a test
for.  Note that the test used a hard-coded value for WSAExxx since there were
(and still are) no constants in Perl for that.  Users will now be able to test
for EINVAL instead, but no existing code will be using that because it has
always had the value 22.

This non-backwards-compatible breakage seems unavoidable if we are to get things
in a sane state for the future because it is really a problem with how things
have long been, namely that $! was being set to winsock2.h values, but if there
was a corresponding errno.h value and it was < 100 then *that* was used for the
Errno/POSIX constant, so authors had no good way of testing for such $! values.
The new scheme corrects that by consistently using the same values for $! and
for Errno/POSIX [except in the case of third-party code setting its own $!/errno
values, which will be addressed in the next commit].

(Ironically, Exxx values >= 100 (the ones which caused all this trouble in the
first place!) don't have this problem as long as code uses the Exxx constants
rather than the hard-coded 100XX numbers.  For example, if code tests $! ==
ECONNABORTED, where $! and ECONNABORTED were both previously set to winsock2.h
values then the same test will still work, except that now $! and ECONNABORTED
are both set to errno.h values in VC10+ (or both left as winsock2.h values upto
VC9).  It does still break if any code is directly testing $! == 10053, but
that's not such a problem: that's really broken code, which should be using the
constant instead.)
Intercept assignment to $! to translate WSAExxx values to Exxx values on Windows

Since perl previously assigned WSAExxx values to $! on Windows it is quite
possible that (Perl-level) user code may also manually make similar assignments,
which will now cause breakage if the value put in $! is subsequently compared to
Errno/POSIX constants because the latter are now the corresponding Exxx values
where possible.

An example of this is in Net::Ping::tcp_connect(), which does the following to
fetch a socket-level error code:

unpack("i", getsockopt($self->{"fh"}, SOL_SOCKET, SO_ERROR))

and assigns the result (a WSAExxx value such as 10061) to $! and then goes wrong
in the subsequent test (in ping_tcp()) for $! == ECONNREFUSED (which is now 107
rather than 10061 if perl is built with VC10 or higher).

To avoid this we now intercept assignment to $! and convert any WSAExxx values
to Exxx values first.  This causes a minor oddity in that this:

perl -le "$! = 10061; print 0+$!"

will now output 107 (for VC10+ perls) but this is surely preferable to the
alternative breakage described above.
Fix stringification of $! in VC10+ builds where errno > sys_nerr

VC++ 2010 and above define a "POSIX supplement" of errno values ranging from
EADDRINUSE (100) to EWOULDBLOCK (140), but sys_nerr is still 43 and strerror()
returns "Unknown error" for them. We already avoid using strerror() if errno
> sys_nerr, but we treat such values as Windows error codes (i.e. <winerror.h>
values) and look up the corresponding system messages for them. There is no
better plan for these POSIX supplement errno values, but they must be
converted from <errno.h> values to <winerror.h> values first otherwise we
will look up the wrong system message. In practice, we only expect to find
errno > sys_nerr in the case of Windows sockets errors (we used to assign
WSAGetLastError() to errno), so we simply convert Exxx values to WSAExxx
values where possible, and use a default <winerror.h> value otherwise (namely,
ERROR_INVALID_FUNCTION).

This change fixes code such as this:

perl -le "$!=107; print $!"

which now outputs the expected "No connection could be made because the
target machine actively refused it." rather than "The program stopped
because an alternate diskette was not inserted." (in VC10+ builds).

Also: Fix the spelling of ECANCELED.
Simplify errno2.h slightly

There is no need for it to worry about ensuring that standard errno.h values
like EINTR are defined.

There is no point in it defining dummy constants like EINVALIDPROCTABLE which
do not exist in errno.h at all. They would only be given the corresponding
WSAExxx values anyway, so the win32sck.c function might just as well return
them directly.

Other constants like ESOCKTNOSUPPORT which do not exist in errno.h on Windows
but presumably exist elsewhere since POSIX tries to exports them are still
worth defining (where possible -- it isn't possible for EHOSTDOWN since there
is no corresponding WSAEHOSTDOWN); add comments for these for clarity.

Also comment which errno.h constants are new in VC10, and mention three of
the four that win32/include/sys/socket.h used to redefine because they are
used in the perl core (ENOTSOCK, EAFNOSUPPORT and ECONNABORTED) -- the other
one (ECONNRESET) does not appear to be used in the perl core as far as I can
tell.
Add more errno.h constants to POSIX for exporting if they exist

This change adds the following constants, which are all found in errno.h in
MS VC++ 2010 and higher:

EBADMSG
ECANCELED
EIDRM
EILSEQ
ENODATA
ENOLINK
ENOMSG
ENOSR
ENOSTR
ENOTRECOVERABLE
ENOTSUP
EOTHER
EOVERFLOW
EOWNERDEAD
EPROTO
ETIME