New command-line option: 'putty --host-ca'.

This causes PuTTY to bring up just the host CA configuration dialog
box, and shut down once that box is dismissed.

I can imagine it potentially being useful to users, but in the first
instance, I expect it to be useful to _me_, because it will greatly
streamline testing changes to the UI of that dialog!

unix/platform.h

@@ -245,6 +245,7 @@ GtkWidget *create_message_box(
     bool selectable, const struct message_box_buttons *buttons,
     post_dialog_fn_t after, void *afterctx);
 #endif
+void show_ca_config_box_synchronously(void);
 
 /* window.c needs this special function in utils */
 int keysym_to_unicode(int keysym);