Allow threads to sleep when available

By mean of an an UCI option it is possible let the available
threads to sleep, this should help with Hyper Threading although
is not the best solution when number of threads equals number
of available cores.

Option is disabled by default.

No functional change.

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

src/ucioption.cpp

@@ -94,6 +94,7 @@ void init_uci_options() {
   Options["Minimum Split Depth"] = Option(4, 4, 7);
   Options["Maximum Number of Threads per Split Point"] = Option(5, 4, 8);
   Options["Threads"] = Option(1, 1, MAX_THREADS);
+  Options["Use Sleeping Threads"] = Option(false);
   Options["Hash"] = Option(32, 4, 8192);
   Options["Clear Hash"] = Option(false, "button");
   Options["Ponder"] = Option(true);