pimd: Cleanup S,GRPt prune handling on Mroute Loss

pimd: Cleanup S,GRPt prune handling on Mroute Loss

1) Clean up display of S,GRPt prune state to be more meaningful
2) Upon receipt of a S,GRPt prune make sure we transition to
   the correct state
3) Upon loss of a S,GRPt prune make sure we transition to
   the correct state as well as immediately send a *,G
   join upstream to propagate the loss of the prune.
4) Removal of a weird S,G state being installed upon
   loss of a S,G RPt prune.

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