pthread_setcanceltype() and pthread_testcancel() do not have to be system calls if cancellation points are not enabled.

Update TODO, cosmetic changes, spelling -- US English spells it canceled and canceling vs cancelled and cancelling.  No idea why.

pthread_mutex_destroy(): Fix an error in destorynig a mutex which can occur after a pthread has been canceled while holding the mutex.

cancellation points are basically function.  More tested is needed and additional cancellation points must be implemented before this can be merged back to master.