Artifact bugfix

 Bugfix: Abandoned artifacts will no longer prevent new copies from
 loading. (MUME's prolonged uptimes have made this issue more important.)

 One side effect of the change is that artifacts carried by players will
 also have more opportunities to crumble. Although the formulas have not
 changed, objects carried by players may now crumble a few hours sooner
 than they would have in the past.