A machine says which networks it routes, and its filter forwards them

The derived filter denies forwarding by default and then allows the container
runtime's two default pools, named in this code with a comment saying a machine
configured otherwise needs to say so -- and no way to say it. So the filter was
right on a machine using the defaults and silently wrong on any other.

Measured today: flipping a workstation to the derived filter cut egress for five
of its container networks and for every network its test beds create, because
those come from ranges the defaults do not cover. Nothing reported a fault; the
guests just could not reach anything, while the machine reported it had applied
what it was told.

A node-level fact beside the public domain, because the machine routes them and
the module that loads the filter may be replaced. Added to the defaults, never
replacing them. Their guests also keep address and name service, without which a
network does not work at all, and the converge preview now says what a machine
routes instead of leaving it to a sentence about what it cannot preview.

cmd/mesh-controller/main.go

@@ -148,6 +148,9 @@ func usage() {
   node public-domain <name>      the domain it composes its routed names under
   node public-domain <name> <d>  ...set it to d
   node public-domain <name> --clear   ...it faces the outside no longer
+  node networks <name>           the networks it routes for what it hosts
+  node networks <name> <cidr>... ...set them; its filter forwards these too
+  node networks <name> --clear   ...only the container runtime's own
   token issue --node <name>      a one-time right to join, for an existing record
   token issue --new <name>       create the record and issue for it
   token issue ... --adopted      ...for a machine in use, which joins adopted