This routine sets the cancelability type and returns the previous
    type in location oldtype.

    When a thread's cancelability state is set to PTHREAD_CANCEL_
    DISABLE, (see pthread_setcancelstate()), a cancelation request
    cannot be delivered to that thread, even if a cancelable routine
    is called or asynchronous cancelability type is enabled.

    When the cancelability state is set to PTHREAD_CANCEL_ENABLE,
    cancelability depends on the thread's cancelability type, as
    follows:

    o  If the thread's cancelability type is PTHREAD_CANCEL_DEFERRED,
       the thread can only receive a cancelation request at a
       cancelation point (including condition waits, thread joins,
       and calls to pthread_testcancel()).

    o  If the thread's cancelability type is PTHREAD_CANCEL_
       ASYNCHRONOUS, the thread can be canceled at any point in its
       execution.

    When a thread is created, the default cancelability type is
    PTHREAD_CANCEL_DEFERRED.

                                 CAUTION

       If the asynchronous cancelability type is set, do not call
       any routine unless it is explicitly documented as "safe for
       asynchronous cancelation." Note that none of the general
       run-time libraries and none of the POSIX Threads libraries
       are safe for asynchronous cancelation except for pthread_
       setcanceltype() and pthread_setcancelstate().

       Use asynchronous cancelability only when you have a compute-
       bound section of code that carries no state and makes no
       routine calls.