Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
With `pthread_cleanup_(push|pop)` you can define a function that should
be ran if the thread is exited while it is inside this scope. A thread
can be ended here through an explicit call to `pthread_exit()`, or if
cancellation was requested and a cancellation point was hit.
`pthread_cleanup_*` is mostly only useful together with cancellation
points, and cancellation points are only useful with a cleanup
functionality. Cancellation points are at least partially implemented by
means of `pthread_testcancel()`.
C.f. ["Cancellation Points"][1].
[1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_05_02
This fixes#755.
The pthread ID cannot be reused as soon as the thread ends, because
another thread needs to join it first. `pthread_self()` uses the native
(i.e. RIOT's) thread ID to distinguish itself. A native thread ID can be
reused as soon as the thread ends, since the core knows no join
operation.
In order to not confuse itself with an earlier zombie thread (i.e a dead
non-detached thread, that was not joined, yet), we need to invalidate
the associated native thread ID.
This approach is sane since a dead thread won't call `pthread_self()`
anymore.
