Handle UCI command "mate in x moves"

Following a user request I added the handling of UCI:

go mate x

Currently we just return from a PV node if x moves have been
done. Probably not the best approach. I have looked at Fruit/Toga
sources and there is even simpler: engine falls back on a fixed
depth search.

No functional change.

src/uci.cpp

@@ -215,6 +215,7 @@ namespace {
         else if (token == "depth")     is >> limits.depth;
         else if (token == "nodes")     is >> limits.nodes;
         else if (token == "movetime")  is >> limits.movetime;
+        else if (token == "mate")      is >> limits.mate;
         else if (token == "infinite")  limits.infinite = true;
         else if (token == "ponder")    limits.ponder = true;
     }