sfxge(4): add API to control UDP tunnel ports

HW needs to know which UDP packets should be treated as tunnel
encapsulation to do inner packet recognition, classification and
offloads.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18134