Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

- Garbage collect argc/argv; bump WARNS to 6
- Make the socket path random and move it out of /tmp as that's outside ATF's
prescribed path

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

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