Declare all dsfs as a dedicated type name 'DSF'.

In this commit, 'DSF' is simply a typedef for 'int', so that the new
declaration form 'DSF *' translates to the same type 'int *' that dsfs
have always had. So all we're doing here is mechanically changing type
declarations throughout the code.

net.c

@@ -462,7 +462,7 @@ static int net_solver(int w, int h, unsigned char *tiles,
     unsigned char *tilestate;
     unsigned char *edgestate;
     int *deadends;
-    int *equivalence;
+    DSF *equivalence;
     struct todo *todo;
     int i, j, x, y;
     int area;