Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Add a test for PR 151758.
- While here, make this compile and work on non-i386:
- Use CMSG_SPACE(), CMSG_LEN(), and CMSG_FIRSTHDR() instead of ignoring
padding between 'struct cmsghdr' and control message payloads.
- Don't initialize the control message before calling recvmsg().
Instead, check that we get a valid control message on return from
recvmsg().
- Use errx() instead of err() for some errors that don't report failures
that set errno.

Requested by: kib (1)

o style.Makefile(5): WARNS= -> WARNS?=.

Add unix_passfd, a simple set of regression tests for UNIX domain socket
file descriptor passing. These tests are not all currently passed.