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]
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50152

(cherry picked from commit a4618caa43c16e6e672c27dac1943fd833f8bd8f)