I would like to support per-FIB multicast routing tables, such that one
can run a routing daemon per-FIB, with each daemon oblivious to the
existence of others. Currently the multicast routing code is completely
unaware of FIBs.

To that end, start collecting various global variables in ip_mroute.c
into a per-VNET struct mfctable. In a future patch this will be
expanded into an array of mfctable structures, one per FIB. For now,
all of the state is collected into V_mfctables[0].

Each mfctable contains, among other things:

• a pointer to the routing socket, if routing is enabled,
• a hash table of routing cache entries,
• an table of network interfaces participating in the routing configuration

This change has no functional effect, it is just factoring out these
global variables to make the subsequent patches simpler.