Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC: 260791

Add a command line argument to turn off blocking waiting for the user
to press Ctrl-C (-b). This allows tests with tight loops of mcgrabs
that can stress the multicast tables.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Moved mcgrab(1) into a separate directory -- our .mk infrastructure
doesn't currently have support for building multiple programs in a
single makefile.

While here, fixed manpages and makefiles (missing dependencies).

Add a new program to the multicast test suite. The mcgrab program
is used to grab and hold some number of multicast addresses in order
to test what happens when an interface goes over the number of multicast
addresses it can filter in hardware.