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.

sys/windows/windmain.c

@@ -300,7 +300,7 @@ _CrtSetReportFile(_CRT_ASSERT, _CRTDBG_FILE_STDERR);*/
     if WINDOWPORT(tty) {
         int i;
 
-        for (i = 0; i < 20; ++i) {
+        for (i = 0; i < 5; ++i) {
             nh_delay_output();
         }