Fix node count range

The node count was still clipped to 32 bit because one of the structs
it passes through (FrontEndProgramStats) had the nodes field declared
as an unsigned long.

frontend.h

@@ -215,7 +215,7 @@ void EnableNamedMenuItem P((char *menuRef, int state));
 typedef struct FrontEndProgramStats_TAG {
     int which;
     int depth;
-    unsigned long nodes;
+    u64 nodes;
     int score;
     int time;
     char * pv;