The LBR$UNMAP_MODULE routine unmaps the module, with the record
    file address in txtrfa, from process P2 space. This action
    releases the resources used to map the module.

    Unlike other LBR services that use RMS services, LBR$UNMAP_MODULE
    also uses system services. Because of this, the secondary status
    for error returns is placed in LBR$GL_SUBSTS. Use this to find
    further status when an error is returned.