sunsword vs gremlin

The original report complained that gremlins seemed impervious to
Sunsword's light yet a flash from a camera caused them to cry out in pain
despite "The long sword named Sunsword begins to shine brilliantly!"

This commit does two things:
1. A dmg bonus is applied against gremlins using a lit Sunsword.
2. Gremlins will generally avoid the light emitted by Sunsword.
There's a few minor flavor bits thrown in also.

It is understood that this effectively makes Sunsword provide
"gremlin-proofing", but the gremlin myth and Sunsword's characteristic
feature pretty much demand it.

bug 42

src/weapon.c

@@ -329,6 +329,8 @@ struct monst *mon;
             bonus += rnd(4);
         if (objects[otyp].oc_material == SILVER && mon_hates_silver(mon))
             bonus += rnd(20);
+        if (artifact_light(otmp) && otmp->lamplit && hates_light(ptr))
+            bonus += rnd(8);
 
         /* if the weapon is going to get a double damage bonus, adjust
            this bonus so that effectively it's added after the doubling */