Rewrite the broken code to move cargo with its carrier

The old code did not move a carrier's cargo (planes, land units,
nukes) when the carrier moved.  Instead, it fixed up the location in
the postread callback.  Anything not going through ef_read(), in
particular the update, saw it in its old, incorrect location, until a
fixed up copy got written back.

Moreover, the timestamp did not change when cargo moved, so
incremental dumps did not pick up the movement.

The new code moves the cargo along with the carrier.

New unit_update_cargo() moves or destroys a carrier's cargo (planes,
land units, nukes) along with the carrier.  Call it from
shp_prewrite(), pln_prewrite() and lnd_prewrite() when the carrier
moves or gets destroyed.

Remove the code to destroy cargo from shp_prewrite(), pln_prewrite(),
lnd_prewrite().

Remove the code to fix up cargo location from pln_postread(),
lnd_postread(), nuk_postread().

This changes the message for ship and land unit cargo getting
destroyed from "sunk" and "MIA" to "lost".

include/unit.h

@@ -56,3 +56,4 @@ extern void unit_list(struct emp_qelem *);
 extern void unit_put(struct emp_qelem *list, natid actor);
 extern char *unit_path(int, struct empobj *, char *);
 extern void unit_view(struct emp_qelem *);
+extern void unit_update_cargo(struct empobj *);