RootMoveList sorting: be compatible with std::sort

sort() and sort_multipv() are almost the same, so
use only one implementation.

Also introduce the natural RootMove::operator<() to
compare the moves instead of compare_root_moves(),
this will allow to use std::sort instead of our
home grown bubble-sort.

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