input: stop a modified Delete key from entering stuff into the buffer

When using option -K on an xterm-like terminal, pressing <Shift+Del>,
<Alt+Del>, or <Ctrl+Del> would enter "2~", "3~", or "5~", respectively,
into the buffer.  Now it will just report an "Unknown sequence".

src/winio.c

@@ -1085,6 +1085,8 @@ int convert_sequence(const int *seq, size_t length, int *consumed)
 							   * Linux console/xterm/Terminal. */
 						if (length > 2 && seq[2] == '~')
 							return KEY_DC;
+						if (length > 4 && seq[4] == '~')
+							*consumed = 5;
 						break;
 					case '4': /* Esc [ 4 ~ == End on VT220/VT320/
 							   * Linux console/xterm. */