Add support for static-group setting per phyint

[jrocha]

I'm using pimd to route multicast packets across different AWS regions over a GRE tunnel.
I've been able to make everything work but group joins/leave. Capturing some packets shows that AWS does propagate IGMP join/leaves.

Is there a way to add something like a static group membership on configuration?

[troglobit]

Hi! There's nothing yet, I did ad the functionality to mrouted -- the progenitor of this code base -- last year. Considering how similar the code bases are, it shouldn't be too hard to add it to pimd as well. However, there are other more pressing matters being worked on right now, so can't make any promises of course.

(Changing issue title to reflect wish list status of issue.)