pimd: Further refinement of switch on a stick

pimd: Further refinement of switch on a stick

The original modification to allow the RP to receive
packets on the incoming interface when it has
no outgoing interfaces was a bit aggressive and
turned all register events into a join event.

This further divides up the code to do the
right thing in each situation.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>