LinuxKPI: add is_unicast_ether_addr()

Needed by mediatek/mt76 wireless driver.

While here adjust the way is_multicast_ether_addr() is implemented
to not look weird. [1]

Sponsored by: The FreeBSD Foundation
Suggested by: emaste [1]
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50152