POSIX port - Cancel and join all FreeRTOS managed pthreads upon shutdown

POSIX port - Cancel and join all FreeRTOS managed pthreads upon shutdown

For a clean shutdown where memory is freed, it is necessary for all pthreads to be joined
at shutdown.

Previously there was explicit cancellation of the idle task and timer daemon task, however
there may be a number of other tasks in the system, both system created and user created,
and those tasks/threads were being left at shutdown.

This change calls pthread_cancel()/pthread_join() on all FreeRTOS managed pthreads upon
shutdown.