"fix" github issue #475 - Trollsbane

Player's pet killed a troll with Trollsbane and the corpse later
revived.  He assumed that killing a troll with Trollsbane is what
prevents troll corpse revival but that is inhibited by the hero
be wielding Trollsbane at the time revival is attempted.

Having killed-by-Trollsbane be the reason for blocking revival
would be much better but looks like a lot of work for something
which was supposed to be a one-line enhancement to an under-used
artifact.  This extends revival inhibition to having anyone on
the level be wielding Trollsbane rather than just the hero.
Not a proper fix but I think it's better than nothing.

Closes #475

src/zap.c

@@ -784,8 +784,7 @@ revive(struct obj *corpse, boolean by_hero)
     }
 
     if ((mons[montype].mlet == S_EEL && !IS_POOL(levl[x][y].typ))
-        || (mons[montype].mlet == S_TROLL
-            && uwep && uwep->oartifact == ART_TROLLSBANE)) {
+        || (mons[montype].mlet == S_TROLL && Trollsbane_wielded())) {
         if (by_hero && cansee(x, y))
             pline("%s twitches feebly.",
                 upstart(corpse_xname(corpse, (const char *) 0, CXN_PFX_THE)));