patch 8.2.1809: mapping some keys with Ctrl does not work properly

Problem:    Mapping some keys with Ctrl does not work properly.
Solution:   For terminal, GTK and Motif handle "@", "^" and "_" codes.

src/gui_x11.c

@@ -956,6 +956,9 @@ gui_x11_key_hit_cb(
     {
 	len = mb_char2bytes(key, string);
 
+	// Some keys need adjustment when the Ctrl modifier is used.
+	key = may_adjust_key_for_ctrl(modifiers, key);
+
 	// Remove the SHIFT modifier for keys where it's already included,
 	// e.g., '(', '!' and '*'.
 	modifiers = may_remove_shift_modifier(modifiers, key);