Save dummy groups to cache during initgroups
    
    If during initgroups operation we find out that any of the groups
    the user is a member of is not cached yet we add a incomplete,
    expired group entry. That way, we save ourselves from looking up and
    saving all the potential user entries the group may also consist of.
    Because the group is expired, it will be refreshed during the next
    getgrgid/getgrnam call and correct member list will be returned.