pimd: add new/distinct enumeration for pim register state

pimd: add new/distinct enumeration for pim register state

With the separation of register-state and upstream-join-state we no
longer need an enumeration that covers both states. This commit includes
the following -
1. Defined new enumeration for reg state (this 1:1 with RFC4601).
2. Dropped JOIN_PENDING enum value from upstream join state. RFC4601
only define two values NOT_JOINED and JOINED for this state.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-14700
Testing Done: Verified register setup manually and ran pim-smoke