pimd: pim_nexthop_lookup should return true/false

pimd: pim_nexthop_lookup should return true/false

The current reverse logic led to this code construct
if (!pim_nexthop_lookup(...)) {
	//Do something successfull
}

This is backwards and will cause logic errors when people
use this code.  Fix to use true/false for success/failure.

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