on_exit_reset fails to clear DSM-related exit actions

I just noticed that the DSM patch has introduced a whole new class of
failures related to the bug #9464 issue: to wit, any on_detach
actions registered in a parent process will also be performed when a
child process exits, because nothing has been added to on_exit_reset
to prevent that.  It seems likely that this is undesirable.
        regards, tom lane