Set timestamp automatically on write

Do it in do_write().  Remove the setting of timestamps elsewhere.

This fixes empdump to set timestamps to the current time instead of
zero on import.

src/server/lostitem.c

@@ -58,7 +58,6 @@ delete_lostitems(void *unused)
 	    if (lost.lost_timestamp > (now - lost_items_timeout))
 		continue;
 	    lost.lost_owner = 0;
-	    lost.lost_timestamp = 0;
 	    putlost(n, &lost);
 	    ncnt++;
 	}