Disable by default the new Copy-on-Write for 5.18

It was felt that the new COW feature wasn't ready to be enabled be default
in 5.18: principally because too much XS code will assume it can just
manipulate the PVX buffer of an SVf_POK SV.

See RT #116569 for the discussion.
Fix declaration after statement in vms.c's Perl_my_chdir

And while we're there, set errno appropriately for the empty string
input case.
Enable sem* functions in configure.com for 8.4+.

They became available in VMS v8.4 in 2010.
Stop SEGV on 'our sub { syntax error }'

Fix for RT #116981. If a sub is declared with our, the name is added
to the stash early, and left with a NULL ptr if there's a syntax error
while compiling it.

Since the only time it becomes an issue is when that same name
is used in the same scope after a syntax error, what happens in
the pad is not particularly important.  The simple fix is to
simply fall back to treating it like a bareword, and pretending
it was never added to the pad in the first place.
deparse.t: mask STDERR noise on lex sub TODO test

A Deparse TODO test which tries to deparse lexical subs, produce a bunch of
"unexpected OP_CLONE" etc on STDERR (since it doesn't handle those new ops
yet). Capture and skip those warnings for now.

Also, add 'no warnings "experimental::lexical_subs"'
to stop a warning about using an experimental feature.
fixed minor typo in delta
Update IPC-Cmd to CPAN version 0.80

  [DELTA]

  Changes for 0.80    Sat Mar  2 22:03:49 GMT 2013
  =================================================
  * Use File::Spec->path instead of relying on
    Config (William Orr)
Configure: bail out when neither stdarg.h nor varargs.h

Perl uses variadic functions, so we need at least one of the relevant
mechanisms available; for this reason, the CompCert C compiler is unsuitable
for our purposes.

If the person building Perl wants to carry on regardless, they can use the
"-K" option.

This partly addresses RT#112494.
regen/unicode_constants.pl: Convert '-' in names to '_'

Unicode character names can have dashes in them.  These aren't accepted
in C macro names.  Change so both blanks and the hyphen-minus are
converted to underscores.
regen/unicode_constants.pl: Pass through input comments

The data can now have comments, which are converted to C and passed
through
regen/unicode_constants.pl: Remove unused constant

This was added in the 5.17 series, so can't be yet in the field; and
isn't needed.
regen/unicode_constants.pl: Make portable to non-ASCII

This now uses the U+ notation to indicate code points, which is
unambiguous not matter what the platform's character set is.  (charnames
accepts the U+ notation)
regen/unicode_constants.pl: Change #define name

This was added in the 5.17 series so there's no code relying on its
current name.  I think that the abbreviation is clearer.