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