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 second pass to the close-before-accept test using a call to the
tcp drop sysctl. This should exercise the ECONNABORTED accept case.

Add a regression test to exercise the case where accept() is called and
the next waiting socket has had a close on the remote endpoint already.