nss: add SSS_NSS_GETORIGBYNAME request
    
    This patch adds a new request to the nss responder which follows the
    same flow as a SSS_NSSGETSIDBYNAME request but returns more data than
    just the SID. The data is returned as pairs of \0-terminated strings
    where the first string is the sysdb attribute name and the second the
    corresponding value.
    
    The main use case is on the FreeIPA server to make additional user and
    group data available to the extdom plugin which then send this data to
    SSSD running on FreeIPA clients.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>