Support ARP for 802 networks

This is used by 802.3 Ethernet. (Also be used by 802.4 Token Bus and
802.5 Token Ring, but we don't support those.)

This was accidentally removed along with FDDI support in commit
0437c8e3b198, presumably because comments implied it was used only by
FDDI or Token Ring.

Fixes: 0437c8e3b198 ("Remove support for FDDI networks.")
Reviewed-by: emaste
Signed-off-by: Denny Page <dennypage@me.com>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1166