use the given user's login group.
If SPEC_ARG contains a `:', then use that as the separator, ignoring
any `.'s. If there is no `:', but there is a `.', then first look
- up SPEC_ARG as a login name. If that look-up fails, then try again
- interpreting the `.' as a separator.
+ up the entire SPEC_ARG as a login name. If that look-up fails, then
+ try again interpreting the `.' as a separator.
USERNAME and GROUPNAME will be in newly malloc'd memory.
Either one might be NULL instead, indicating that it was not