pimd: provide a mechanism to pin the IIF for an SG entry

 pimd: provide a mechanism to pin the IIF for an SG entry

In the case of vxlan origination entries IIF is set to -
1. lo for single VTEPs
2. MLAG-ISL for VTEPs multihomed via MLAG.

This commit creates the necessary infrastructure by -
1. allowing the IIF to be set statically (without RPF lookup)
2. and by preventing next-hop-tracking registration

PS: Note that I have skipped additional checks in pim_upstream_del
intentionally i.e. an attempt will be made to remove nexthop-tracking
for the upstream entry (with STATIC_IIF) which will fail because of the
up-entry not being in the nh's hash table. Ideally we should maintain
a nh pointer in the up-entry to prevent this unnecessary processing.
In the abscence of that I wanted to avoid spraying STATIC_IIF checks
all over.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>