Assorted trivia in search.cpp

The only interesting change is the moving of
stack[MAX_PLY+4] back to its original position
in id_loop (now renamed Thread::search).

No functional change.

src/search.h

@@ -103,8 +103,8 @@ extern LimitsType Limits;
 extern StateStackPtr SetupStates;
 
 void init();
-void reset();
-template<bool Root> uint64_t perft(Position& pos, Depth depth);
+void clear();
+template<bool Root = true> uint64_t perft(Position& pos, Depth depth);
 
 } // namespace Search