IFP: Add a GetGroupsList method
    
    This patch adds a new method on the bus with the following synopsis:
    <method name="GetUserGroups">
        <arg name="user" type="s" direction="in" />
        <arg name="values" type="as" direction="out"/>
    </method>
    
    Its purpose is to return names of groups the user is a member of as a
    list of strings.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit 3fe339bcba0e211cc666bb3afe34e5c8fce85f4f)