Add option to create actual system users/groups

[Dave Shoreman]

While it's technically possible currently by manually setting the user/group id to something in the "system" range, it's not exactly convenient and you run the risk of clashing with an existing user/group.

For updates, manual is the only way to achieve it but when we're creating a new user or group we can utilise the --system arg telling it to use an id from the system range, thus neglecting the need to manually set an id.

---

groupadd

   -r, --system
       Create a system group.

       The numeric identifiers of new system groups are chosen in the
       SYS_GID_MIN-SYS_GID_MAX range, defined in login.defs, instead of
       GID_MIN-GID_MAX.

groupmod

   does not apply

useradd

   -r, --system
       Create a system account.

       System users will be created with no aging information in /etc/shadow, and
       their numeric identifiers are chosen in the SYS_UID_MIN-SYS_UID_MAX range,
       defined in /etc/login.defs, instead of UID_MIN-UID_MAX (and their GID
       counterparts for the creation of groups).

       Note that useradd will not create a home directory for such a user,
       regardless of the default setting in /etc/login.defs (CREATE_HOME). You
       have to specify the -m options if you want a home directory for a system
       account to be created.

usermod

   does not apply