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/flag.h

@@ -246,6 +246,8 @@ struct instance_flags {
                             * to input becoming taken over);
                             * True => enable fuzzer when entering moveloop */
     boolean herecmd_menu;  /* use menu when mouseclick on yourself */
+    boolean idlecheckpoint;  /* platform should perform a checkpoint update
+                              * if waiting for input longer than 10 seconds */
     boolean invis_goldsym; /* gold symbol is ' '? */
     boolean in_lua;        /* executing a lua script */
     boolean lua_testing;   /* doing lua tests */