passfd: fix incorrect sendmsg arguments The unit test hung on NetBSD, which pointed out a couple of bugs. * lib/passfd.c (sendfd): Avoid uninitialized msg_flags field, and incorrect msg_controllen value. * modules/passfd-tests (Depends-on): Check for alarm. * tests/test-passfd.c (main) [HAVE_DECL_ALARM]: Avoid hanging test. Reported by Bastien ROUCARIES. Signed-off-by: Eric Blake <eblake@redhat.com>