Allow pf to use dummynet pipes and queues.

We re-use the currently unused IPFW_IS_DUMMYNET flag to allow dummynet
to tell us that a packet is being re-injected after being delayed. This
is needed to avoid endlessly looping the packet between pf and dummynet.

MFC after:	2 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D31904