Add net/dual-dhclient-daemon port.

This serves the same purpose as net/dual-dhclient, but does it
better: While net/dual-dhclient is a trivial shell script which
launches two dhclients, this is a daemon which
(a) can be signalled by /etc/rc.d/dhclient stop, and
(b) relays that signal to the two "child" dhclient daemons.

This is deliberately not replacing the net/dual-dhclient port
since in the unlikely event that something is broken I don't
want to see pkg automatically upgrading people; but EC2 AMIs
will be making use of this new port and I intend to remove the
old dual-dhclient port at some point.