sfxge(4): provide option to disable not a local MAC address check

Option EFSYS_OPT_ALLOW_UNCONFIGURED_NIC disables check that the adapter
MAC address is not a local address (beginning 02).

Submitted by: Laurence Evans <levans at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6508