feat(stop): warn players in-game before stop, restart and update, and retry postponed updates (#5025)
* feat(stop): warn players in-game before stop, restart and update, and retry postponed updates
* fix(stop): exit when players prevent a stop during update or backup, keep module modes
* fix(stop): mark variables shared between modules for shellcheck
* refactor(stop): shorten check_players_online module description
* refactor(stop): keep chat commands out of the config, clean up locks when a stop is cancelled, exit on CTRL-C
* feat(stop): say why the player count is unavailable
* fix(monitor): run postponed updates in the background, keep a long update's lockfile, retry forced updates as forced
* fix(monitor): mark exitcode as used by core_exit for shellcheck