PAM: Export original shell to tlog-rec-session
    
    Add exporting of original user shell (as returned by NSS) as an
    environment variable for use by tlog-rec-session, when session recording
    is enabled for the user. This lets tlog-rec-session start the actual
    user shell, after tlog-rec-session is started in its place.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>