(PUP-2454) Use 999 as the max system uid for certain operating systems.

User ID's below 1000, not 500, are generally considered system users on
OpenBSD, FreeBSD, and Debian operating systems.

Users can still modify this with 'uid_start' in adduser.conf, but in that
case they'd need to provide an actual value instead of 'true'.

Also fixing the fact that 500 should not be included as a system ID.
The inclusive maximum should be 499 for other operating systems.
(PUP-2454) Add spec examples for testing unless_system_user special casing.

Adding examples to test for handling unless_system_user being
automatically set to 999 on certain operating systems.

Changing an existing example to ensure that user 500 is purged
correctly.
Merge branch 'pr/2736'

* pr/2736:
  (PUP-2454) Add spec examples for testing unless_system_user special casing.
  (PUP-2454) Use 999 as the max system uid for certain operating systems.