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/misc2.c

@@ -2946,6 +2946,32 @@ find_special_key(
 }
 
 
+/*
+ * Some keys are used with Ctrl without Shift and are still expected to be
+ * mapped as if Shift was pressed:
+ * CTRL-2 is CTRL-@
+ * CTRL-6 is CTRL-^
+ * CTRL-- is CTRL-_
+ * Also, <C-H> and <C-h> mean the same thing, always use "H".
+ * Returns the possibly adjusted key.
+ */
+    int
+may_adjust_key_for_ctrl(int modifiers, int key)
+{
+    if (modifiers & MOD_MASK_CTRL)
+    {
+	if (ASCII_ISALPHA(key))
+	    return TOUPPER_ASC(key);
+	if (key == '2')
+	    return '@';
+	if (key == '6')
+	    return '^';
+	if (key == '-')
+	    return '_';
+    }
+    return key;
+}
+
 /*
  * Some keys already have Shift included, pass them as normal keys.
  * Not when Ctrl is also used, because <C-H> and <C-S-H> are different.