Improve namespacing in <sys/socket.h>:

 * MSG_NOSIGNAL is in POSIX.1-2008.
 * MSG_NOTIFICATION (SCTP) is not in POSIX.
 * PRU_FLUSH_* (SCTP) are not in POSIX.
 * bindat()/connectat() are not in POSIX.

Discussed with:	rrs (PRU_FLUSH_*)