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

Correct several nits/problems in the unix_close_race regression test.

Submitted by: Mikolaj Golub <to.my.trociny at gmail.com>
MFC after: 3 days

Add unix_close_race, a regresion test to catch ENOTCONN being returned
improperly from one of two instances of close(2) being called
simultaneously on both ends of a connected UNIX domain socket. The test
tool is slightly tweaked to improve failure modes, and while often does
trigger the problem, doesn't do so consistently due to the nature of the
race.

PR: kern/144061
Submitted by: Mikolaj Golub <to.my.trociny@gmail.com>
MFC after: 3 days