sys_un-h: Ensure that <sys/un.h> defines sa_family_t.
* lib/sys_un.in.h: Include <sys/socket.h> or define sa_family_t
explicitly.
* m4/sys_socket_h.m4 (gl_PREREQ_SYS_SA_FAMILY): New macro, extracted
from gl_SYS_SOCKET_H.
(gl_SYS_SOCKET_H): Invoke it.
(gl_SYS_SOCKET_H_DEFAULTS): Don't initialize HAVE_SA_FAMILY_T.
* m4/sys_un_h.m4 (gl_SYS_UN_H): Invoke gl_PREREQ_SYS_SA_FAMILY. Test
whether <sys/un.h> defines sa_family_t. If not, set GL_GENERATE_SYS_UN_H
to true.
* modules/sys_un-h (Makefile.am): Substitute HAVE_SA_FAMILY_T,
HAVE_SA_FAMILY_T_IN_SYS_UN_H.
* tests/test-sys_un-h.c: Include intprops.h. Verify that sa_family_t is
defined and an unsigned integer type.
* modules/sys_un-h-tests (Depends-on): Add intprops.
* doc/posix-headers/sys_un.texi: Mention the problem with sa_family_t.