Harden CARP against network loops.

If there is a loop in the network a CARP that is in MASTER state will see it's
own broadcasts, which will then cause it to assume BACKUP state. When it
assumes BACKUP it will stop sending advertisements. In that state it will no
longer see advertisements and will assume MASTER...

We can't catch all the cases where we are seeing our own CARP broadcast, but
we can catch the obvious case.

Submitted by: torek
Obtained from: FreeNAS
MFC after: 2 weeks
Sponsored by: iXsystems