Allow to specify port with --devtools option (fixes #3597)

Note that using `servo --devtools http://example.org` doesn't work. In
that case either the port must be specified or the option moved to the
end. This is done by getopts and is the same for other such options,
e.g. `--profile`.

ports/cef/core.rs

@@ -68,7 +68,7 @@ pub extern "C" fn cef_run_message_loop() {
         show_debug_borders: false,
         enable_text_antialiasing: true,
         trace_layout: false,
-        devtools_server: false,
+        devtools_port: None,
         initial_window_size: TypedSize2D(800, 600),
         user_agent: None,
     };