sys_socket: Ensure ss_family field on AIX. * lib/sys_socket.in.h (ss_family): New macro definition. * m4/sys_socket_h.m4 (gl_HEADER_SYS_SOCKET): Set HAVE_STRUCT_SOCKADDR_STORAGE_SS_FAMILY. Set SYS_SOCKET_H if necessary. (gl_SYS_SOCKET_H_DEFAULTS): Initialize HAVE_STRUCT_SOCKADDR_STORAGE_SS_FAMILY. * modules/sys_socket (Makefile.am): Substitute HAVE_STRUCT_SOCKADDR_STORAGE_SS_FAMILY. * doc/posix-headers/sys_socket.texi: Mention the AIX bug.