Remove size parameter from dsf init and copy functions.

Now that the dsf knows its own size internally, there's no need to
tell it again when one is copied or reinitialised.

This makes dsf_init much more about *re*initialising a dsf, since now
dsfs are always allocated using a function that will initialise them
anyway. So I think it deserves a rename.

tents.c

@@ -2267,7 +2267,7 @@ static int *find_errors(const game_state *state, char *grid)
      * start of the game, before the user had done anything wrong!)
      */
 #define TENT(x) ((x)==TENT || (x)==BLANK)
-    dsf_init(dsf, w*h);
+    dsf_reinit(dsf);
     /* Construct the equivalence classes. */
     for (y = 0; y < h; y++) {
 	for (x = 0; x < w-1; x++) {