If no group was specified, but a group by the same name as the user
exists, an error was raised in the situation where USERGROUPS_ENAB is
enabled in /etc/login.defs (which is the case for almost every major
linux distro). In this case, the user will be put in group 100 (which
is usually the "users" group on those same distros). This is currently
only done in the base class, as the issue may not exist on other
platforms like AIX or the BSDs.

Fixes #6210