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 regression test for listen()'s backlog argument, both at time of
creation and at time of update using an additional call to listen().
This test also exercises SO_LISTENQLIMIT, a forthcoming socket option
that allows the retrieval (but not setting) of the queue limit.

Discussed with: andre