Fix update to take dead units off carriers

Fix update to take dead units off carriers

upd_plane() upd_land() and left planes and land units lost to lack of
maintenance on their carriers.  Cargo lists were fine anyway, because
unit_cargo_init() ignored dead units.  But when the dead unit got
reused for building a new one, pln_prewrite() / lnd_prewrite() got
confused and attempted to take it off its carrier, which made
clink_rem() oops, because the unit wasn't on the cargo list.  No real
harm done, as oops recovery was fine.

Fix upd_plane() and upd_land() to clear the carrier.  Make
unit_cargo_init() oops when it finds dead units on carriers.