Actually make DSF an opaque structure type.

This makes good on all the previous preparatory commits, which I did
separately so that each one individually has a reasonably readable
diff, and all the mechanical changes are separated out from the
rewrites that needed actual thought.

Still no functional change, however: the DSF type wraps nothing but
the same int pointer that 'DSF *' used to store directly.

puzzles.h

@@ -426,7 +426,7 @@ char *button2label(int button);
 /*
  * dsf.c
  */
-typedef int DSF;
+typedef struct DSF DSF;
 DSF *snew_dsf(int size);
 void dsf_free(DSF *dsf);