ignitable() macro

ignitable() was excluding magic lamp and then every place that
used it did so as 'ignitable(obj) || obj->otyp == MAGIC_LAMP'
so just include magic lamp.

I noticed that while hunting for an explanation for report #K2734
where returning to a previously visited level triggered the
warning "begin_burn: unexpected eggs".  I've decided that the
zombie apocalypse is probably the cause.  It inserted a new type
of timer in the list of such but it didn't bump EDITLEVEL to
invalidate save and bones files which relied on indices into the
old list.  I'm not sure whether we should bump that now.

src/light.c

@@ -1,4 +1,4 @@
-/* NetHack 3.7	light.c	$NHDT-Date: 1596498173 2020/08/03 23:42:53 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.45 $ */
+/* NetHack 3.7	light.c	$NHDT-Date: 1604442297 2020/11/03 22:24:57 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.46 $ */
 /* Copyright (c) Dean Luick, 1994                                       */
 /* NetHack may be freely redistributed.  See license for details.       */
 
@@ -691,8 +691,7 @@ boolean
 obj_is_burning(obj)
 struct obj *obj;
 {
-    return (boolean) (obj->lamplit && (obj->otyp == MAGIC_LAMP
-                                       || ignitable(obj)
+    return (boolean) (obj->lamplit && (ignitable(obj)
                                        || artifact_light(obj)));
 }