New puzzle: `Slant', picked from the Japanese-language section of

nikoli.co.jp (which has quite a few puzzles that they don't seem to
have bothered to translate into English).

Minor structural change: the disjoint set forest code used in the
Net solver has come in handy again, so I've moved it out into its
own module dsf.c.

[originally from svn r6155]

list.c

@@ -31,6 +31,7 @@ extern const game pegs;
 extern const game rect;
 extern const game samegame;
 extern const game sixteen;
+extern const game slant;
 extern const game solo;
 extern const game twiddle;
 extern const game untangle;
@@ -50,6 +51,7 @@ const game *gamelist[] = {
     &rect,
     &samegame,
     &sixteen,
+    &slant,
     &solo,
     &twiddle,
     &untangle,