[faux-mgs] Add --sp-sim-addr CLI flag

[jgallagher]

Trying to use faux-mgs to talk to sp-sim running on localhost fails in a confusing way where sp-sim gets packets from faux-mgs, but faux-mgs rejects the response because they come back with a scope ID of 0:

$ faux-mgs --interface lo --discovery-addr '[::1]:33300' discover
Feb 06 22:59:04.587 INFO creating SP handle on interface lo, component: faux-mgs
Feb 06 22:59:04.588 WARN failed to look up interface for peer; discarding packet, error: no interface name found for index 0, peer: [::1]:33300, component: faux-mgs

I think this is because scope IDs are only provided for link-local addresses? The behavior here is the same on Linux and illumos.

This PR adds a new flag specifically for talking to the simulator, which goes through the test path we created for MGS to do this same thing:

$ faux-mgs --sp-sim-addr '[::1]:33300' discover
Feb 07 19:19:05.363 INFO creating SP handle on to talk to SP simulator at [::1]:33300, component: faux-mgs
Feb 07 19:19:05.363 INFO initial discovery complete, addr: [::1]:33300, component: faux-mgs
Feb 07 19:19:05.363 INFO SP discovered, port: One, addr: [::1]:33300, interface: (direct socket handle), component: faux-mgs
addr=[::1]:33300, port=One