Walls of lava

Add "walls of lava", basically lava which blocks vision and
require a bit more than just levitation or flight to move through.

No levels use this yet, as testing isn't thorough enough.

src/dbridge.c

@@ -65,7 +65,7 @@ is_lava(coordxy x, coordxy y)
     if (!isok(x, y))
         return FALSE;
     ltyp = levl[x][y].typ;
-    if (ltyp == LAVAPOOL
+    if (ltyp == LAVAPOOL || ltyp == LAVAWALL
         || (ltyp == DRAWBRIDGE_UP
             && (levl[x][y].drawbridgemask & DB_UNDER) == DB_LAVA))
         return TRUE;