pimd: Tracking of RPF is *separate* from the lookup

pimd: Tracking of RPF is *separate* from the lookup

Start the separation of tracking a Destination from the act
of looking it up.  The cojoining of these two concepts led
to a bunch of code that had to think about both problems leading
to weird situations and code paths.  Simplify the code by making
pim_ecmp_nexthop_search a static function and we only ever
call pim_ecmp_nexthop_lookup when we need to do a RPF().

pim_ecmp_nexthop_lookup will now attempt to find a stored pnc
and if it finds one it will report on the answer from it.

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