sysusers: take configuration as positional arguments

sysusers: take configuration as positional arguments

If the configuration is included in a script, this is more convient.
I thought it would be possible to use this for rpm scriptlets with
'%pre -p systemd-sysuser "..."', but apparently there is no way to pass
arguments to the executable ($1 is used for the package installation count).
But this functionality seems generally useful, e.g. for testing and one-off
scripts, so let's keep it.

There's a slight change in behaviour when files are given on the command line:
if we cannot parse them, error out instead of ignoring the failure. When trying
to parse all configuration files, we don't want to fail even if some config
files are broken, but when parsing a list of items specified explicitly, we
should.

v2:
- rename --direct to --inline