Eliminate pre-5.9.x conditional code for PERL_PACK_CAN_BYTEORDER

PERL_PACK_CAN_BYTEORDER has been unconditionally defined for versions 5.9.x
and greater, and undefined for 5.8.x. As we are never going to need to
port changes back to maint-5.8 any more, eliminate all the 5.8.x related code,
and the macro that supports it.
Eliminate pre-5.9.x conditional code for PERL_PACK_CAN_SHRIEKSIGN

PERL_PACK_CAN_SHRIEKSIGN has been unconditionally defined for versions 5.9.x
and greater, and undefined for 5.8.x. As we are never going to need to
port changes back to maint-5.8 any more, eliminate all the 5.8.x related code
and the macro that supports it.
Eliminate the definitions for DO_BO_PACK_P and DO_BO_UNPACK_P.

Commit 07409e015252427f in April 2005 replaced all the uses of these two
macros with DO_BO_PACK_PC and DO_BO_UNPACK_PC.
vtohl, vtohs, htovl and htovs are no-ops on 64 bit little endian systems.

Previously they were implemented as function calls on 64 bit little endian
systems. Bit endian systems implemented them as byte-swapping macros. 32
little endian system didn't implement them at all. 32 and 64 bit little
endian systems now behave identically.
Provide vtohl, vtohs, htovl and htovs no-op macros on little endian systems.

This means that there are always macros or functions for vtohl, vtohs,
htovl and htovs available, so eliminate HAS_VTOHL etc, and unconditionally
compile the code that it was protecting. grep.cpan.me shows that no code on
CPAN uses any of these macros.

(Technically the 4 are not quite no-ops, as they truncate their values to
32 or 16 bits, to be consistent with the implementations for platforms which
need re-ordering.)
Provide ntohl, ntohs, htonl and htons no-op macros on big endian systems.

This means that there are always macros or functions for ntohl, ntohs,
htonl and htons available, so eliminate use of HAS_NTOHL etc, and
unconditionally compile the code that it was protecting. However, as code
on CPAN is using these guard macros, define all of them in perl.h

(Technically the 4 are not quite no-ops, as they truncate their values to
32 or 16 bits, to be consistent with the implementations for platforms which
need re-ordering.)
Fallbacks for ntohl, ntohs, htonl and htons for little endian systems.

The host byteorder agnostic functions in util.c are now only used on mixed
endian systems.
Update the Win32 miniperl conditional compilation, for the htonl etc changes.

As of commit 19253ae62cd13079 (Oct 2012), miniperl on Win32 avoids using
Winsock. The win32_* wrappers for htonl etc had used the pre-processor
macro MYSWAP to conditionally compile in the correct code. However, MYSWAP
was defined as a side effect of using the htonl etc wrappers in util.c,
which are no longer needed. Hence use the WIN32_NO_SOCKETS macro directly
in win32sck.c for the correct conditional compilation.
Remove buggy loop-based byte swapping code.

The irony is that the union-based code special-cased for little endian
systems actually works everywhere, even on mixed-endian systems.
Eliminate the conditionally-compiled fallback functions for htonl etc.

These are now only being used for mixed-endian platforms which do not
provide their own htnol (etc) functions. Given that the fallbacks have been
buggy since they were added in Perl 3.0, it's safe to conclude that no
mixed-endian platforms were ever using these functions.

It's also unclear why these functions were ever marked as 'A', part of the
API. XS code can't call them directly, as it can't rely on them being
compiled. Unsurprisingly, no code on CPAN references them.
Refactor macros in pp_pack.c, removing support for mixed-endian byte orders.

The byte-order handling can be simplified considerably if we don't have to
support mixed-endian systems. It's not clear whether Perl 5.000 even
compiled on PDP-11s, let alone more recent versions.

Support probably can be added back at the end of the refactoring, if someone
has time and a PDP-11 to test on, but for the intermediate stages it is a
lot easier not to need to think about such platforms.
In pp_pack.c, refactor DO_BO_(UN)?PACK_N to use sizeof() on the variable.

Previously they were using sizeof() on the type argument passed to the
macro. This change makes their implementation more like DO_BO_(UN)?PACK
and DO_BO_(UN)?PACK_PTR.
In pp_pack.c, refactor DO_BO_(UN)?PACK_PTR to use my_letohn etc

Previously they were casting the pointer to an integer value, calling
a my_letoh* function for the appropriate size, and casting the return value
back to a pointer.
In pp_pack.c, refactor DO_BO_(UN)?PACK to use my_letohn etc

Previously they were using a my_letoh* function for the appropriate size.

This commit probably breaks fixed 16 and 32 bit templates on big endian
Crays where sizeof(short) and sizeof(int) are 8. A future commit will
restore support. (Although it's not clear whether such Crays can still
build blead cleanly, as we've not had any feedback since Boeing
decommissioned their Cray.)
Eliminate my_{hto[bl]e,[bl]etoh}{16,32,64,s,i,l} as nothing now uses them.
Use DO_BO_{UN,}PACK in place of DO_BO_UNPACK_{N,PC}

Also, eliminate the intermediate macros DO_BO_PACK_PTR and DO_BO_UNPACK_PTR.
DO_BO_{UN,}PACK can use my_swabn() directly.

This removes the only users of my_{be,le,h}to{be,le,h}n, so remove the
definitions for those macros.
Swap byte order in DO_BO_(UN)?PACK based on a variable needs_swap.

Add the macro NEEDS_SWAP to initialise needs_swap based on
TYPE_ENDIANNESS(datumtype). This makes the two definitions of DO_BO_UNPACK
identical, and the two definitions of DO_BO_PACK identical.

This also makes building pp_pack.c on a mixed endian byteorder architecture
a compile time error. The commit adds pointers on where to add code to
re-instate support for such architectures.
Merge the identical definitions of DO_BO_UNPACK and DO_BO_PACK.

Delete the now-unused mixed-endian definitions of DO_BO_UNPACK and
DO_BO_PACK, and the macro BO_CANT_DOIT, as they were the last two things
using it.
As DO_BO_(UN)?PACK are now implemented with my_swabn(), simplify 'J' and 'j'.

The conditionally compiled code to figure out the correct named integer
size for UVs and IVs is no longer needed, as my_swabn() will swap UVs and
IVs, whatever their length.