netstat: Add -F support for -g

This change adds the ability to examine the contents of multicast
routing tables for other FIBs without the need for executing
netstat with setfib(1).

MFC after: 3 days
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D56205
Reviewed by: glebius, markj, zlei