Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Rework TIMEWAIT regression test so that kernel-allocated port numbers are
used rather than a fixed userspace one, avoiding conflicts between the two
test runs.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.

When running the second part of the test, kill off the server process from
the first part before starting, or the TCP port we want to bind may be in
use still. Sleep for a short period between tests.

Use SIGTERM instead of SIGKILL.

Pass the right pid into the parent and child tasks so that when a test
fails, we kill the right partner process.

Add a short regression test that triggers a TCP socket to enter time wait
while the socket is still open; one variant closes the socket while in
time wait, and a second lets time wait expire while the socket is open.