Make special levels generate objects with genocided monster classes

If a special level explicitly requests eg. a statue with a genocided
monster class, allow generating it.

Rationale is that those objects were generated before the monsters
became extinct. Also fixes a lua error.

include/monflag.h

@@ -203,5 +203,6 @@ enum ms_sounds {
 #define MV_KNOWS_EGG                        \
     0x0008 /* player recognizes egg of this \
               monster type */
+#define G_IGNORE 0x1000 /* for mkclass(), ignore G_GENOD|G_EXTINCT */
 
 #endif /* MONFLAG_H */