Change formula for computing min book weight from selectivity setting,
to play a wider variety of opening moves.
change to clock_gettime for timing on Linux.
Use faster sampling for very short time controls.
Modify sampling frequency for short time controls. Use timeGetTime
instead of GetTickCount for timing on Windows. Make interval variables
long.
Merge branch 'timing' - increase sample freq for very short games.
(mostly effective for Linux since Windows is still using lower-res
timer). + introduce CLOCK_TYPE for portability.