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.

unfinished/separate.c

@@ -331,7 +331,7 @@ static void solver_init(struct solver_scratch *sc)
      * contents array, however, because this will change if we
      * adjust the letter arrangement and re-run the solver.
      */
-    dsf_init(sc->dsf, wh);
+    dsf_reinit(sc->dsf);
     for (i = 0; i < wh; i++) sc->size[i] = 1;
     memset(sc->disconnect, 0, wh*wh * sizeof(bool));
 }