With DECC$ALLOW_UNPRIVILEGED_NICE enabled, the nice function
    exhibits its legacy behavior of not checking the privilege of
    the calling process (that is, any user may lower the nice value
    to increase process priorities). Also, when the caller sets a
    priority above MAX_PRIORITY, the nice value is set to the base
    priority.

    With DECC$ALLOW_UNPRIVILEGED_NICE disabled, the nice function
    conforms to the X/Open standard of checking the privilege of the
    calling process (only users with ALTPRI privilege can lower the
    nice value to increase process priorities), and when the caller
    sets a priority above MAX_PRIORITY, the nice value is set to MAX_
    PRIORITY.