Allow flipping levels horizontally or vertically

When a special level is created, there's a chance it gets flipped
horizontally and/or vertically.

Add new level flags "noflip", "noflipx", and "noflipy" to prevent
flipping the level. Add a wiz-mode command #wizlevelflip to test
the flipping on current level - although this doesn't flip everything,
as level flipping is meant to happen during level creation.

include/sp_lev.h

@@ -86,6 +86,7 @@ struct sp_coder {
     int n_subroom;
     int lvl_is_joined;
     boolean check_inaccessibles;
+    int allow_flips;
 };
 
 /*