passfd: speed up configure and drop unused code

passfd: speed up configure and drop unused code

Ultimately, it would be nice to provide a sendmsg and recvmsg module,
and have those provide a replacement struct msghdr and silently
convert msg_accrights into the replacement struct's msg_control, when
targeting older BSD.  But until that point in time, this is a nice
cleanup.

SCM_RIGHTS really only works on Unix sockets; however, there is
nothing in passfd.c that needs to be hard-coded to this (rather,
the hard-coding of the address family is done externally when
the socket or socketpair is created).

* m4/afunix.m4 (gl_SOCKET_AFUNIX): Rename...
* m4/passfd.m4 (gl_PASSFD): ...to something more fitting of
its use.  Drop test for setting unused HAVE_UNIXSOCKET_SCM_RIGHTS.
Instead of probing at configure for unix_scm_rights_bsd44_way,
instead probe for CMSG_FIRSTHDR at compile time.  Simplify BSD 4.3
check to a struct member probe.
* lib/passfd.c (includes): Nothing here requires <sys/un.h>.
(sendfd, recvfd): Update preprocessor checks.
* modules/passfd (Files): Reflect rename, and drop unused file.
(Depends-on): Drop unused dependency.

Signed-off-by: Eric Blake <eblake@redhat.com>