Detach search arguments from UI thread

Detach from the UI thread the input arguments used by
the search threads so that the UI thread is able to receive
and process any command sent by the GUI while other threads
keep searching.

With this patch there is no more need to block the UI
thread after a "stop", so it is a more reliable and
robust solution than the previous patch.

No functional change.

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

src/search.h

@@ -54,6 +54,7 @@ namespace Search {
 
 struct LimitsType {
 
+  LimitsType() {  memset(this, 0, sizeof(LimitsType)); }
   bool useTimeManagement() const { return !(maxTime | maxDepth | maxNodes | infinite); }
 
   int time, increment, movesToGo, maxTime, maxDepth, maxNodes, infinite, ponder;
@@ -66,7 +67,7 @@ struct SignalsType {
 extern volatile SignalsType Signals;
 extern LimitsType Limits;
 extern std::vector<Move> RootMoves;
-extern Position* RootPosition;
+extern Position RootPosition;
 
 extern void init();
 extern int64_t perft(Position& pos, Depth depth);