`mutex_unlock()` did not restore IRQsbefore yielding for the woken up
thread.

* should not have any effect as long as `unsigned` and `int` are compatible
* also fix two cosmetic `unsigned int` -> `unsigned` for consistency

Fixes #1708.

Currently involuntary preemption causes the current thread not only to
yield for a higher prioritized thread, but all other threads of its own
priority class, too.

This PR adds the function `thread_yield_higher()`, which will yield the
current thread in favor of higher prioritized functions, but not for
threads of its own priority class.

Boards now need to implement `thread_yield_higher()` instead of
`thread_yield()`, but `COREIF_NG` boards are not affected in any way.

`thread_yield()` retains its old meaning: yield for every thread that
has the same or a higher priority.

This PR does not touch the occurrences of `thread_yield()` in the periph
drivers, because the author of this PR did not look into the logic of
the various driver implementations.

Rename queue to priority queue, because that's what it is.

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.

There is no need to supply the current priority to `sched_switch()`,
when this function can easily tell the value of
`active_thread->priority` itself.

Also changed names for bitarithm functions and rename thread_pid to sched_active_pid.

- Included a collection of cpu-dependent headers in core/include/arch
- Extracted all interfaces that need to be implemented for a cpu
- Created a mapping between those interfaces and the old ones
- added flag for disabling arch interface
- added missing state to lpm_arch interface
- added arch interface for reboot
- fixed newline issues that were pointed out
- documentation fixes to cpu-core interface

sched_switch can check ISR itself.

C++11 requires a space between
literal and identifier [-Wliteral-suffix]

kaspar.schleiser@fu-berlin.de is obsolete.
(2nd try, first try was overwritten by some overzealous documenter)

kaspar.schleiser@fu-berlin.de will be obsoleted soon. Replace it with
kaspar@schleiser.de, which will (hopefully) stay.