Introduce bridge_ifinput() to handle some repeated logic before

Introduce bridge_ifinput() to handle some repeated logic before
if_input() and to have a counterpart for bridge_ifenqueue() that helps
to understand the traffic/code flow in bridge better.  The bridge
currently only puts a single packet on the input mbuf list, and
changing will need to undo part of this commit, but it still makes
sense to have a well-defined call for the ports receive path.
No functional change.

OK mpi@