Turn the proxy type radio buttons into a dropdown list.

This makes room to add more entries without the Proxy panel
overflowing. It also means we can put in a bit more explanation in
some of the more cryptic one-word names!

putty.h

@@ -2584,6 +2584,13 @@ void conf_fontsel_handler(union control *ctrl, dlgparam *dlg,
 void setup_config_box(struct controlbox *b, bool midsession,
                       int protocol, int protcfginfo);
 
+/* Visible outside config.c so that platforms can use it to recognise
+ * the proxy type control */
+void proxy_type_handler(union control *ctrl, dlgparam *dlg,
+                        void *data, int event);
+/* And then they'll set this flag in its generic.context.i */
+#define PROXY_UI_FLAG_LOCAL 1 /* has a local proxy */
+
 /*
  * Exports from bidi.c.
  */