net: move IsReachable() code to netbase and encapsulate it

`vfLimited`, `IsReachable()`, `SetReachable()` need not be in the `net`
module. Move them to `netbase` because they will be needed in
`LookupSubNet()` to possibly flip the result to CJDNS (if that network
is reachable).

In the process, encapsulate them in a class.

`NET_UNROUTABLE` and `NET_INTERNAL` are no longer ignored when adding
or removing reachable networks. This was unnecessary.

src/torcontrol.cpp

@@ -409,7 +409,7 @@ void TorController::get_socks_cb(TorControlConnection& _conn, const TorControlRe
         //
         // If NET_ONION is not reachable, then none of -proxy or -onion was given.
         // Since we are here, then -torcontrol and -torpassword were given.
-        SetReachable(NET_ONION, true);
+        g_reachable_nets.Add(NET_ONION);
     }
 }