Unix PuTTY: add missing call to prepare_session.

Now 'putty user@host' will do what you wanted on Unix the same way it
always has on Windows.

(Thanks to Geoff Winkless for pointing out this inconsistency. I've
redone his actual patch my way, but he should still be credited for
the inspiration!)

unix/gtkwin.c

@@ -4597,6 +4597,8 @@ void new_session_window(Conf *conf, const char *geometry_string)
 {
     struct gui_data *inst;
 
+    prepare_session(conf);
+
     /*
      * Create an instance structure and initialise to zeroes
      */