Consistently use peer label to ID peers when enforcing split horizon

Consistently use peer label to ID peers when enforcing split horizon

We were incorrectly comparing the peer label (which is "hostname:port")
with the GSS name of our route's source (i.e., "credential@apc.x") when
checking whether we were about to advertise a route back to the trust
router that announced it to us. That broke split horizon enforcement.