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

Fix a fd leak in socketpair():
- Close the new file objects created during socketpair() if the copyout of
the new file descriptors fails.
- Add a test to the socketpair regression test for this edge case.

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

Use WARNS and rely on rules rather than an explicit target for the
test binary.

Reported by: maxim

Start the dreaded NOFOO -> NO_FOO conversion.

OK'ed by: core

For variables that are only checked with defined(), don't provide
any fake value.

Add minimal socketpair() regression test to confirm that we can create
(and close) PF_UNIX socket pairs, and that we can't create PF_INET
socket pairs. More tests to follow.