Fix sending of best move during an infinite search
According to UCI standard once engine receives 'go infinite'
command it should search until the "stop" command and do not exit
the search without being told so, even if PLY_MAX has been reached.
Patch is quite invasive because it cleanups some hacks used
by fixed depth and fixed nodes modes, mainly during benchmarks.
Bug found by Pascal Georges.
Signed-off-by: Marco Costalba <mcostalba@gmail.com>