add option idlecheckpoint

New experimental option

This requires platform support to be useful. Currently an
implementation for Windows TTY console is included.

A Unix TTY implementation should be achievable I would think,
but I haven't pursued that, at least not yet (contributions welcome).
I'm not sure whether Qt or X11 interfaces offer a similar
timeout capability..

How it works:

When idlecheckpoint in on, if the wait for a new input is idle for
10 seconds (length of time to waite is controlled by
IDLECHECKPOINT_WAIT_TIME #define in hack.h), save_currentstate() is
called to bring everything up-to-date should a hangup or crash occur.
The input wait then continues/resumes.

The save_currentstate() call is only executed once per input request.

include/config.h

@@ -681,6 +681,16 @@ typedef unsigned char uchar;
 #endif
 #endif
 
+/* experimential; if the platform/window-port supports it; when the game has
+ * started to wait for player input, and the wait lasts longer than
+ * IDLECHECKPOINT_WAIT_TIME seconds (defined in hack.h or *conf.h), the game
+ * will perform an update to the checkpoint file.
+ * Currently has support in:
+ *     WIN32CON
+ */
+
+/* #define IDLECHECKPOINT */
+
 /* End of Section 4 */
 
 #ifdef TTY_TILES_ESCCODES