Rewrite the logic around the dymanic array of routing tables to help

Rewrite the logic around the dymanic array of routing tables to help
turning rtable_get(9) MP-safe.

Use only one per-AF array, as suggested by claudio@, pointing to an
array of pointers to the routing table heads.

Routing tables are now allocated/initialized per-AF.  This will let
us allocate routing table on-demand instead of always having an
AF_INET, AF_MPLS and AF_INET table as soon as a new rtableID is used.

This also get rid of the "void ***" madness.

ok dlg@, jmatthew@