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 manual page and a Makefile.

Add code to reflect packets back from the sink so that we can measure
round trip at the source.

A multicast test. The mctest program acts as both a source and a sink
for multicast packets. Parameters for the interface, packet size,
number of packets, and interpacket gap may be given on the command line.
The sink records how many packets were missed, and at what time each
packet arrived.