Add test if the message queue is fair
Change reboot signature.

Change from `void reboot(void)` to `int reboot(int mode)`.
Move reboot definition to core, rename architecture implementations
from reboot to reboot_arch.
Declare reboot mode(s) in kernel.h, reboot_arch in kernel_internal.h
Currently only one reboot mode is handled, its use is enforced.

Rationale:
A reboot function is already defined in <unistd.h> on BSD systems.
(See: http://www.openbsd.org/cgi-bin/man.cgi?query=reboot&sektion=2)
This patch not only allows native to build sensibly on these systems
but also streamlines RIOTs compatability with existing software.
Merge pull request #822 from LudwigOrtmann/reboot_signature

core: change reboot signature.
posix: move posix semaphore in posix module
rename test for posix semaphore
correct copyright and doxygen
Merge pull request #866 from Kijewski/add-test-queue-fairness

Add test if the message queue is fair
Merge pull request #845 from mehlis/sem-to-posix

posix: move posix semaphore in posix module