Put a timestamp into struct emptypedstr

Make sure all members of unit empobj_storage share it.

Add matching timestamp member to struct comstr, struct empobj, struct
gamestr, struct lonstr, struct natstr, struct nwsstr, struct trdstr,
struct trtstr.  The timestamp isn't yet set for these.  To be fixed.

Move the timestamp member to the right place in struct lndstr, struct
loststr, struct realmstr, struct nukstr, struct plnstr, struct sctstr,
struct shpstr.

include/empobj.h

@@ -57,6 +57,7 @@ struct empobj {
      */
     short ef_type;
     short uid;
+    time_t timestamp;
     /* end of part matching struct emptypedstr */
     natid own;		/* valid if EFF_OWNER is in table's flags */
     coord x;		/* valid if EFF_XY    is in table's flags */