pimd: Fix code to not send joins when SGRPT Prune is received

pimd: Fix code to not send joins when SGRPT Prune is received

When we receive a SGRPT Prune we were switching the upstream
to JOINED and immediately sending a join.  This was not
the right thing to do.

This was happening because we were making decisions about the
new ifchannel before it was fully formed.

Rework ifchannel startup to provide enough information to
the pim upstream data structure to make the right decisions

Ticket: CM-16425
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>