genl: add a monitor subcommand

usage:
$ genl monitor <family> <multicats group>

this subcommand allows to monitor the message from a multicast group
of a given family when received.

If it knows how to parse the messages received it will dump the decoded
version, otherwise it will just inform a new message has been received

So far it only knows how to parse nlctrl notify messages, but the plan
to allow to make the parsing extensible via lua scripts

Differential Revision: https://reviews.freebsd.org/D40372

(cherry picked from commit e19b2ef976d326d34b17331c5071d9e687d0d5d5)