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

Always assign WARNS using ?=

- fix some nearby style bugs
- include Makefile.inc where it makes sense and reduces duplication

Approved by: ed (co-mentor)

Add unix_sorflush, a regression test for the following scenario:

- Process (a) is blocked in read on a socket waiting on data.
- Process (b) is blocked in shutdown() on a socket waiting on (a).
- Process (c) delivers a signal to (b) interrupting its wait.

When the signal is delivered, the kernel panics as sblock() fails in
sorflush(). Even if it didn't panic, shutdown() would block potentially
indefinitely waiting for recv() to succeeded. Fixes to follow.

Reported by: Jos Backus <jos at catnook dot com>