Increase MultiPV to 5 with Skill

When playing with strength handicap we use MultiPV search
to retrieve a set of possible moves.

This patch increase default multi PV search to 5 from 4 so
to consider a wider set of possible move.

We expect to lower strength because an extra weaker move enters
the set.

This patch is aprerequisite for further work.

No functional change.

src/search.cpp

@@ -323,7 +323,7 @@ void Thread::search() {
   // When playing with strength handicap enable MultiPV search that we will
   // use behind the scenes to retrieve a set of possible moves.
   if (skill.enabled())
-      multiPV = std::max(multiPV, (size_t)4);
+      multiPV = std::max(multiPV, (size_t)5);
 
   multiPV = std::min(multiPV, rootMoves.size());