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

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

Properly initialise the "len" argument to getsockname(2) in the tcpdrop
regression test so that it works (more) consistently.

Approved by: re (bz)
Sponsored by: Juniper Networks

In the tcpdrop regression test, allow the kernel to allocate us a port
rather than using a fixed port number. This means that the regression test
can be run many times in a row without waiting on TIMEWAIT to release a
hard-coded port number.

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

Add regression test for tcpdrop sysctl.