nss_check_name_of_well_known_sid() improve name splitting
    
    Currently in the default configuration
    nss_check_name_of_well_known_sid() can only split fully-qualified names
    in the user@domain.name style. DOM\user style names will cause an error
    and terminate the whole request.
    
    With this patch both styles can be handled by default, additionally if
    the name could not be split nss_check_name_of_well_known_sid() returns
    ENOENT which can be handled more gracefully by the caller.
    
    Resolves https://fedorahosted.org/sssd/ticket/2717
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>