pimd: allow mroutes with IIF in the OIL

pimd: allow mroutes with IIF in the OIL

This is specifically needed to allow pim-evpn mroutes in the MLAG setup -
(36.0.0.11, 239.1.1.100)   Iif: peerlink.4094   Oifs: uplink-1, peerlink.4094

I could have gone the other way and disabled PIM_ENFORCE_LOOPFREE_MFC but
that opens the door too wide. Relaxing the checks for mlag-specific mroutes
seemed like the safer choice.

This commit provides the infrastructure to relax checks on a per-mroute
basis.

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