kill.2, sigaction.2, sigpending.2, sigprocmask.2, sigsuspend.2, ctermid.3, ctime...

kill.2, sigaction.2, sigpending.2, sigprocmask.2, sigsuspend.2, ctermid.3, ctime.3, ferror.3, flockfile.3, fopen.3, getaddrinfo.3, getgrnam.3, getnameinfo.3, getpwnam.3, rand.3, sigsetops.3, sigwait.3, strtok.3, tzset.3, unlocked_stdio.3, feature_test_macros.7: Remove mention of the obsolete _POSIX_SOURCE macro from SYNOPSIS

_POSIX_SOURCE was a POSIX.1-1990 creation that was soon made
obsolete bu _POSIX_C_SOURCE. Retaining mention of it
in the feature test macro requirements section of the
SYNOPSIS doesn't contain important information, and may
mislead readers into actually trying to use this macro.
A few mentions of it a maintained in a some pages where
defining _POSIX_SOURCE inhibits some behavior.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>