o Fix a silly requirement for <machine/signal.h> to be included in the

o Fix a silly requirement for <machine/signal.h> to be included in the
  middle of this header.
o Remove unneeded conditionals to hide SIG* in the POSIX case.
  (C allows implementations to define additional SIG* constants.)
o Add comments about missing features.
o Move the location of the sigset_t typedef.
o Update standards visibility conditionals.
o Fix some assumptions about what pid_t and uid_t are defined as.
o Remove size_t typedef and use __size_t in struct sigaltstack
  instead.