sys_socket: Use POSIX compatible declarations on OSF/1 5.1. * m4/sys_socket_h.m4 (gl_HEADER_SYS_SOCKET): On OSF/1, define _POSIX_PII_SOCKET. * doc/posix-functions/recv.texi: Document the OSF/1 problem. * doc/posix-functions/recvfrom.texi: Likewise. * doc/posix-functions/send.texi: Likewise. * doc/posix-functions/sendto.texi: Likewise.