Reorganise the dsf API into three kinds of dsf.

This is preparing to separate out the auxiliary functionality, and
perhaps leave space for making more of it in future.

The previous name 'edsf' was too vague: the 'e' stood for 'extended',
and didn't say anything about _how_ it was extended. It's now called a
'flip dsf', since it tracks whether elements in the same class are
flipped relative to each other. More importantly, clients that are
going to use the flip tracking must say so when they allocate the dsf.

And Keen's need to track the minimal element of an equivalence class
is going to become a non-default feature, so there needs to be a new
kind of dsf that specially tracks those, and Keen will have to call it.

While I'm here, I've renamed the three dsf creation functions so that
they start with 'dsf_' like all the rest of the dsf API.

pearl.c

@@ -350,7 +350,7 @@ static int pearl_solve(int w, int h, char *clues, char *result,
      * We maintain a dsf of connected squares, together with a
      * count of the size of each equivalence class.
      */
-    dsf = snew_dsf(w*h);
+    dsf = dsf_new(w*h);
     dsfsize = snewn(w*h, int);
 
     /*
@@ -1577,7 +1577,7 @@ static bool check_completion(game_state *state, bool mark)
      * same reasons, since Loopy and Pearl have basically the same
      * form of expected solution.
      */
-    dsf = snew_dsf(w*h);
+    dsf = dsf_new(w*h);
 
     /* Build the dsf. */
     for (x = 0; x < w; x++) {