ipfw: Introduce dnctl

ipfw: Introduce dnctl

Introduce a link to the ipfw command, dnctl, for dummynet configuration.
dnctl only handles dummynet configuration, and is part of the effort to
support dummynet in pf.

/sbin/ipfw continues to accept pipe, queue and sched commands, but these can
now also be issued via the new dnctl command.

Reviewed by:	donner
MFC after:	2 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D30465