NSS: Move nss_get_shell_override to responder utils
    
    Move nss_get_shell_override to common responder utils and rename it to
    sss_resp_get_shell_override to make it available to other responders. In
    particular let PAM responder use it to provide original shell when it is
    overriden for session recording.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>