l2_multi: Minor cleanup
l2_multi: Be more conservative about MAC moves

l2_multi was previously willing to re-learn MAC addresses at a port which
is part of a "half-known" (in one direction only) link.  It should now
only learn them if the link is *entirely* unknown, which is probably a
better idea.

Colin found this case with STS.
l2_multi: Cleanup

This improves the comments for, cleans up, and improves behavior for a
particular case.  (Hopefully.)