pimd: Only create and bind the autorp socket when really needed

pimd: Only create and bind the autorp socket when really needed

Previously, the autorp socket would get created and bind if needed
by autorp configuration.
This update limits it further to also require pim enabled interfaces
in the vrf before the socket is created and bind.
So now the socket will automatically close if there are no pim
enabled interfaces left, or if autorp is turned off. It will
automatically turn on if autorp is turned on and there are pim
enabled interfaces in the vrf.

Signed-off-by: Nathan Bahr <nbahr@atcorp.com>