sfxge(4): support filters for encapsulated packets

This supports filters which match all unicast or multicast
inner frames in VXLAN, GENEVE, or NVGRE packets.
(Additional fields to match on can be added easily.)

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18073