pf: bridge-to

Allow pf (l2) to be used to redirect ethernet packets to a different
interface.

The intended use case is to send 802.1x challenges out to a side
interface, to enable AT&T links to function with pfSense as a gateway,
rather than the AT&T provided hardware.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37193