Look at /etc/login.defs for the system_max_[ug]id values

Look at /etc/login.defs for the system_max_[ug]id values

It makes little sense to make the boundary between systemd and user guids
configurable. Nevertheless, a completely fixed compile-time define is not
enough in two scenarios:
- the systemd_uid_max boundary has moved over time. The default used to be
  500 for a long time. Systems which are upgraded over time might have users
  in the wrong range, but changing existing systems is complicated and
  expensive (offline disks, backups, remote systems, read-only media, etc.)
- systems are used in a heterogenous enviornment, where some vendors pick
  one value and others another.
So let's make this boundary overridable using /etc/login.defs.

Fixes #3855, #10184.