posix: Add pthread_cleanup handlers

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
pthread_cleanup: better documentation
pthread_cleanup: fixup for #798
make: replace findstring with filter in sys/Makefile
Merge pull request #986 from Kijewski/a-minor-fix-for-issue-672-not-everything-but-better-than-nothing-i-guess-or-what-do-you-think

make: replace findstring with filter in sys/Makefile
Merge pull request #821 from Kijewski/pthrad_cleanup

posix: Add pthread_cleanup handlers
Use subfolders in bin dir

Creating all object files in one directory is bound to produce name
clashes. RIOT developers may take care to use unique file names, but
external packages surely don't.

With this change all the objects of a module (e.g. `shell`) will be
created in `bin/$(BOARD)/$(MODULE)`.

I compared the final linker command before and after the change. The
`.o` files (e.g. `startup.o`, `syscall.o` ...) are included in the same
order. Neglecting the changed path name where the `.o` files reside, the
linker command stays exactly the same.

A major problem could be third party boards, because the location of the
`startup.o` needs to the specified now in
`boards/$(BOARD)/Makefile.include`, e.g.
```Makefile
export UNDEF += $(BINDIR)msp430_common/startup.o
```