Make g.coder->lvl_is_joined a boolean

Its value is only used as a boolean, so there's no real need to keep it
as a confusing int.

Shouldn't be a save-breaking change; it doesn't look like g.coder is
saved.

include/sp_lev.h

@@ -91,7 +91,7 @@ struct sp_coder {
     struct mkroom *tmproomlist[MAX_NESTED_ROOMS + 1];
     boolean failed_room[MAX_NESTED_ROOMS + 1];
     int n_subroom;
-    int lvl_is_joined;
+    boolean lvl_is_joined;
     boolean check_inaccessibles;
     int allow_flips;
 };