sfxge: use MAC spoofing TX and MAC change privileges

Update of common code to provide a query on the MAC_SPOOFING_TX and
CHANGE_MAC privileges instead of the deprecated MAC_SPOOFING privilege.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4436