Changes to support building with _POSIX_SOURCE set to 199309L:
1. Add sys/_posix.h to handle those preprocessor defs that POSIX
says have effects when defined before including any header files;
2. Change POSIX4_VISIBLE back to _POSIX4_VISIBLE
3. Add _POSIX4_VISIBLE_HISTORICALLY for pre-existing BSD features now
defined in POSIX. These show up when:
_POSIX_SOURCE and _POSIX_C_SOURCE are not set or
_POSIX_C_SOURCE is set >= 199309L
and vanish when:
_POSIX_SOURCE is set or _POSIX_C_SOURCE is < 199309L.
4. Explain these in man 9 posix4;
5. Include _posix.h and conditionalize on new feature test.
