Combine increased LMR horizont and fixed null move reduction

Set null move reduction to R=4, but increase the LMR horizon
to 3 plies. The two tweaks are related and should compensate
the combined effect of null move + LMR reduction at shallow
depths.

Idea from Tord.

After 999 games at 1+0

Mod vs Orig  +251 =522 -225 51.30% + 9 ELO

On Tord iMac Core 2 Duo 2.8 GHz, one thread,
Mac OS X 10.6, at 1+0 time control we have:

Mod vs Orig 994-1006  -1.4 ELO

But Orig version is pgo compiled and Mod is not.
The PGO compiled version is about 8% faster, which
corresponds to about 7 Elo points. This means that
results are reasonably consistent.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>