This introduces new settings for the users and groups services.
"group_substring_filter_type" for the group services and
"user_substring_filter_type" for the users service. They allow to set
the type of LDAP filter that is used for substring user searches.
Possible values are: "initial", "final" and "any" to do either prefix,
suffix or full substring searches.
Fixes#547
The help flag is configured automatically by default already. We don't
need to redo that for every single service.
This also addresses one of the finding of "go race" (#4088)
