Use slightly lower polling frequency in the last few seconds.

Instead of checking the time every 100 nodes in the last second,
and every 1000 nodes in the last five seconds, Stockfish now checks
every 1000 nodes in the last second and every 5000 nodes in the last
five seconds.  This was tested in 1036 games at a time control of
40 moves/10 seconds, and no losses on time occured.

Also fixed a bug pointed out by Marco:  In infinite mode, myTime
is actually 0, but of course we still don't want to check the time
more frequently than the standard once per 30000 nodes in this
case.