more steadfast

Make changes similar to the suggested patch from entrez:  support
for 'youmonst' as the monster passed to m_carrying().  This doesn't
change carrying(otyp) to call m_carrying(&g.youmonst,otyp) though.

Also, treat being on the Plane of Air or in an air bubble on the
Plane of Water similar to flying or levitating:  wielded Giantslayer
(or carried loadstone) doesn't prevent knockback there.

src/invent.c

@@ -1285,15 +1285,17 @@ nxtobj(struct obj *obj, int type, boolean by_nexthere)
     return otmp;
 }
 
+/* return inventory object of type 'type' if hero has one, otherwise Null */
 struct obj *
 carrying(int type)
 {
     register struct obj *otmp;
 
+    /* this could be replaced by 'return m_carrying(&g.youmonst, type);' */
     for (otmp = g.invent; otmp; otmp = otmp->nobj)
         if (otmp->otyp == type)
-            return  otmp;
-    return (struct obj *) 0;
+            break;
+    return otmp;
 }
 
 /* Fictional and not-so-fictional currencies.