Option to log proxy setup diagnostics to the terminal.

It has three settings: on, off, and 'only until session starts'. The
idea of the last one is that if you use something like 'ssh -v' as
your proxy command, you probably wanted to see the initial SSH
connection-setup messages while you were waiting to see if the
connection would be set up successfully at all, but probably _didn't_
want a slew of diagnostics from rekeys disrupting your terminal in
mid-emacs once the session had got properly under way.

Default is off, to avoid startling people used to the old behaviour. I
wonder if I should have set it more aggressively, though.

config.c

@@ -2072,6 +2072,15 @@ void setup_config_box(struct controlbox *b, int midsession,
 		     HELPCTX(proxy_command),
 		     conf_editbox_handler,
 		     I(CONF_proxy_telnet_command), I(1));
+
+	ctrl_radiobuttons(s, "Print proxy diagnostics "
+                          "in the terminal window", 'r', 5,
+			  HELPCTX(proxy_main),
+			  conf_radiobutton_handler,
+			  I(CONF_proxy_log_to_term),
+			  "No", I(FORCE_OFF),
+			  "Yes", I(FORCE_ON),
+			  "Only until session starts", I(AUTO), NULL);
     }
 
     /*