pimd: Fix RP shenanigans

pimd: Fix RP shenanigans

The RP was not properly handling the series of events:

1) When a WRVIFWHOLE is received if we are the RP,
send a pim register stop to the FHR.

2) When a register was received we were sending
a join (S,G) towards the S, then a immediate prune (S,G)
followed by another join (S,G).  Just send the first join

3) Save whom we received the S,G register from so we
can use it later

4) Allow a join timer to restart itself instead
of causing a crash.

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