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.

src/decl.c

@@ -37,6 +37,8 @@ NEARDATA struct obj *uwep, *uarm, *uswapwep,
     *uarmc, *uarmh, *uarms, *uarmg,*uarmf, *uamul,
     *uright, *uleft, *ublindf, *uchain, *uball;
 
+struct engr *head_engr;
+
 #ifdef TEXTCOLOR
 /*
  *  This must be the same order as used for buzz() in zap.c.