Teach SEE about pinned pieces

Teach SEE about pinned pieces

Remove pinned pieces from attacks when calculating
SEE value.

Algorithm is not perfect, there should be no false
positives, but can happen that we miss to remove a
pinned piece. Currently we don't cach 100% of cases,
but is a tradeoff between speed and accuracy. In any
case we stay on the safe side, so we remove an attacker
when we are sure it is pinned.

About only 0,5% of cases are affected by this patch, not
a lot given the hard work: this is a difficult patch!

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