SAMV7: MCAN: Prevent Interrupt-Flooding of ACKE when not connected to CAN-BUS. An Acknowledge-Error will occur every time no other CAN Node acknowledges the message sent. This will also occur if the device is not connected to the can-bus. The CAN-Standard declares, that the Chip has to retry a given message as long as it is not sent successfully (or it is not cancelled by the application). Every time the chip tries to resend the message an Acknowledge-Error-Interrupt is generated. At high baud rates this can lead in extremely high CPU load just for handling the interrupts (and possibly the error handling in the application). To prevent this Interrupt-Flooding we disable the ACKE once it is seen as long we didn't transfer at least one message successfully.

i.MX6 interrupt handling:  Additional logic needed to handle nested interrupts when an interrupt stack is used

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.