thus it's easier to compare files when testing a tool

While this may cause a few unnecessary getpwnam/getgrnam calls
this makes the API much simpler for using externally and means
we cannot be called with bogus information for u/gid, homedir
and shell and such like.

mga#14346

This is similar to the is_real_user() API added in the previous
commit and will be useful in higher level code which might
need to filter the display appropriately.

Here we use the heuristic that if the GID is in the range 500-999
then we check to see whether the group is the primary group of a
user with the same name (and the user is considered a 'real' user)
or that the group has any member who is considered 'real'.

mga#14346

The first assigned uid has now changed to 1000 (from 500)
to fall in line with most other distros.

This number seems hardcoded in a few places to try to
do a little bit of refactoring and add a new exported
function, is_real_user(), to try and centralise this
logic a little.

This should ultimately fix the likes of drakboot's
autologin user list.

mga#14346