bindings: when Ctrl+Shift+Delete has no keycode, don't use KEY_BSP

When curses gives no code for Ctrl+Shift+Delete, do not fall back
to KEY_BACKSPACE, because then ^H and/or <Backspace> get bound to
'cutwordleft'.

This fixes https://savannah.gnu.org/bugs/?54642.

Bug was introduced with version 3.0, commit e6429e78.

src/nano.c

@@ -2580,7 +2580,7 @@ int main(int argc, char **argv)
 	controlhome = get_keycode("kHOM5", CONTROL_HOME);
 	controlend = get_keycode("kEND5", CONTROL_END);
 	controldelete = get_keycode("kDC5", CONTROL_DELETE);
-	controlshiftdelete = get_keycode("kDC6", KEY_BACKSPACE);
+	controlshiftdelete = get_keycode("kDC6", CONTROL_SHIFT_DELETE);
 #ifndef NANO_TINY
 	/* Ask for the codes for Shift+Control+Left/Right/Up/Down. */
 	shiftcontrolleft = get_keycode("kLFT6", SHIFT_CONTROL_LEFT);

src/nano.h

@@ -588,6 +588,7 @@ enum
 #define SHIFT_CONTROL_DOWN 0x414
 #define SHIFT_CONTROL_HOME 0x415
 #define SHIFT_CONTROL_END 0x416
+#define CONTROL_SHIFT_DELETE 0x417
 #define ALT_LEFT 0x421
 #define ALT_RIGHT 0x422
 #define ALT_UP 0x423