nss: return user_attributes in origbyname request
    
    To allow IPA clients to offer special attributes of AD users form
    trusted domain the extdom plugin on the IPA server must send them to the
    clients. The extdom plugin already uses sss_nss_getorigbyname() to get
    attributes like the SID and the user principal name. This patch adds the
    attributes given by the NSS/IFP user_attributes option to the list of
    attributes returned by sss_nss_getorigbyname().
    
    Fixes https://fedorahosted.org/sssd/ticket/2464
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>