ofp-util: OpenFlow 1.0 can match IPv6 Ethertype even though not L3 or L4.

OpenFlow 1.0 can match on flows that have the IPv6 Ethertype, but
ofputil_usable_protocols() incorrectly reported that such a match required
NXM or OXM.  This commit fixes the problem.

Also, add some related tests.

Reported-by: Nagi Reddy Jonnala <njonnala@Brocade.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>