Cleanups from James H: a few missing statics, a precautionary cast

or two, a debugging fix, a couple of explicit initialisations of
variables that were previously read uninitialised, and a fix for a
whopping great big memory leak in Slant owing to me having
completely forgotten to write free_game().

[originally from svn r6159]

dsf.c

@@ -4,6 +4,8 @@
  * worry about avoiding closed loops.
  */
 
+#include "puzzles.h"
+
 int dsf_canonify(int *dsf, int val)
 {
     int v2 = val;