posix: pthread: implement non-standard try-join and timed-join

These functions can be used to join pthreads in a non-standard way.
The function pthread_tryjoin will not block and simply test whether the
thread has exited already. The function pthread_timed_join will only block
until the specified time. The functions are wrappers for calling the
k_thread_join with timeout K_NO_WAIT and with a specific timeout as opposed
to calling it with K_FOREVER.

Signed-off-by: Cla Galliard <clamattia@gmail.com>