Change slave_available() API

To return a pointer to the available
thread instead of a bool. This allows
to simplify the core loop in split().

No functional change.

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

src/search.cpp

@@ -1025,7 +1025,7 @@ split_point_start: // At split points actual search starts from here
       // Step 19. Check for splitting the search
       if (   !SpNode
           &&  depth >= Threads.minimumSplitDepth
-          &&  Threads.slave_available(thisThread)
+          &&  Threads.available_slave(thisThread)
           &&  thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
       {
           assert(bestValue < beta);