tweak lua gc

lua C++ bindings require ~400KB worth of tables now; so bump memory
available to rt-safe scripts (full interpreter) to 2MB.

Also switch to incremental GC.

libs/lua/lua/luastate.h

@@ -34,6 +34,8 @@ public:
 	int do_command (std::string);
 	int do_file (std::string);
 	void collect_garbage ();
+	void collect_garbage_step ();
+	void tweak_rt_gc ();
 
 	sigc::signal<void,std::string> Print;