Modify escape of ^ key so it can be used as well as Ctrl+^

Fixes FS#721

This makes Ctrl+^ and ^ bindable again as different keys. We do this
by escaping single `^` as `^-`, which is not a valid control character
(unlike `^^`)

The original approach suggested in FS#721 is insufficient, it will
break bindings such as `meta-^` because Irssi is convinced that `^`
introduces a Control-key ("key combo") so it is waiting for what may
follow.

src/fe-text/gui-readline.c

@@ -393,8 +393,8 @@ static void sig_gui_key_pressed(gpointer keyp)
 	}
 
 	if (strcmp(str, "^") == 0) {
-		/* change it as ^^ */
-		str[1] = '^';
+		/* change it as ^-, that is an invalid control char */
+		str[1] = '-';
 		str[2] = '\0';
 	}