Whitelist additional read-only getters for untrusted connections

ruTorrent queries these commands for its settings and status pages.
They are all read-only getters with no side effects, safe to expose
for untrusted SCGI connections.

Tested against ruTorrent with both httprpc and multirpc plugins on
servers with active torrents — all modes (list, settings, totals,
open connections) work with zero blocked commands.

src/command_tracker.cc

@@ -180,4 +180,6 @@ initialize_command_tracker() {
   rpc::rpc.mark_safe("dht.override_port");
   rpc::rpc.mark_safe("dht.add_node");
   rpc::rpc.mark_safe("dht.statistics");
+  rpc::rpc.mark_safe("trackers.numwant");
+  rpc::rpc.mark_safe("trackers.use_udp");
 }