regcomp.c: Remove a useless use of I_STDARG
regcomp.c: Remove a #undef op
regcomp.c: Remove a cargo-culted #undef SPSTART
regcomp.c: Remove the last usage of NO_UNARY_PLUS

This was set for some VMS variants in 5.005, but has since been
excised from vmsish.h.  The ifdef in regcomp was a leftover.
Remove a '#undef ff_next' remnant from ages past.

This was discussed long ago on the list but never actually removed:
http://www.nntp.perl.org/group/perl.perl5.porters/2001/01/msg29492.html
toke.c: Remove a cargo-culted #undef CLINE
Removed an '#ifdef COMMENTARY' in toke.c

This was added as a micro-optimiaztion twenty years ago.
Remove HAS_64K_LIMIT

This was only defined for MSDOS if not using DJGPP.  We've long
since dropped support for that, so this define and related code
can go.
Remove an ifdef for the Harris HCX-9 froms sv.c

Historical cruft.
Remove the ifdefs for ULTRIX_STDIO_BOTCH

Not only has Ultrix been long out of support, this ifdef was
working around a bug particular to Ultrix 1.2.
Removed the ifdef for FPUTS_BOTCH

This enabled a workaround for fputs on SunOS 4.0.1 and 4.0.2,
SunOS 4.1.x and later do not have the problem.
Removed the ifdef & define for VDf

This remained only for compatibility with CPAN, but nothing there
uses it.
Removed the ifdefs for __SC__ in toke.c
Removed OP_IN_REGISTER and related defines.

Added as an experiment in 462e5cf6, it never quite worked, and
recently wasn't even using registers.
perl.h: Always unconditionally include sys/types.h
Replaced an ifdef for sv_dup with USE_ITHREADS
Removed the ifdefs for BUGGY_MSC and BUGGY_MSC6

These are leftovers from the perl 4 era config.h.
Removed DUMP_FDS and dump_fds()

If perl was compiled with -DDUMP_FDS, it would define dump_fds
and add it to the API, although even then nothing used it.
dump_fds() itself was buggy, only checking for fds 0 through 32.
Replaced the last use of HAS_GNULIBC with __GLIBC__
Removed the use of SPARC64_GCC_WORKAROUND

As the name implies, this worked around a bug in gcc, particularly,
gcc 2.x, in SPARC64.  No longer relevant.