pimd, pim6d: Send register msg via register socket

pimd, pim6d: Send register msg via register socket

The problem here is when the same node is FHR as well as RP,
then the node keeps on sending the register packet.
Register-stop is not sent as well.

This problem has occurred because the RP is the same node
and there is no socket created on loopback interface, so the
packet is never send out and never received back on the same
node, so register recv could not be processed on the node and
hence no register-stop is sent.

Since register packets are unicast packets, its better to handle
the send of register packet via a separate register socket.
This fixes the problem mentioned above as well.

Fixes: #11331
Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>