CLEANUP: Remove obsolete OS/arch code, assume modern POSIX, fix linker errors

CLEANUP: Remove obsolete OS/arch code, assume modern POSIX, fix linker errors

Remove support for dead platforms (Apollo, ISC202, OSF/1, PCS/MUNIX,
Windows NT/95, DEC Alpha) and obsolete compatibility shims (K&R prototypes,
VOIDSIG, OPT_TYPE, valloc, sys_errlist, custom strtok/strerror/inet_netof).

Assume POSIX throughout: signals via sigaction, non-blocking via O_NONBLOCK,
standard headers (errno.h, unistd.h, stdlib.h, string.h, stdarg.h),
getrusage for resource stats, lrand48 for random numbers.

Fix pre-existing multiple-definition linker errors by changing bare global
declarations in headers (h.h, common.h) to extern and placing the actual
definitions in ircd.c.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>