As discussed in #2725, this commit renames a number of stacksize constants to
better convey their intended usage. In addition, constants for thread priority
are given a `THREAD_` prefix. Changes are:

* KERNEL_CONF_STACKSIZE_PRINTF renamed to THREAD_EXTRA_STACKSIZE_PRINTF
* KERNEL_CONF_STACKSIZE_DEFAULT renamed to THREAD_STACKSIZE_DEFAULT
* KERNEL_CONF_STACKSIZE_IDLE renamed to THREAD_STACKSIZE_IDLE
* KERNEL_CONF_STACKSIZE_MAIN renamed to THREAD_STACKSIZE_MAIN
* Move thread stacksizes from kernel.h to thread.h, since the prefix changed
* PRIORITY_MIN renamed to THREAD_PRIORITY_MIN
* PRIORITY_IDLE renamed to THREAD_PRIORITY_IDLE
* PRIORITY_MAIN renamed to THREAD_PRIORITY_MAIN
* Move thread priorities from kernel.h to thread.h since the prefix has changed
* MINIMUM_STACK_SIZE renamed to THREAD_STACKSIZE_MINIMUM for consistency

To avoid naming conflicts + made variable static and volatile as it
should be.

As @authmillenon pointed out the "null" in the old name is somewhat
misleading, since the actual value is -1.

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.

insert missing "is"

however, this won't help to completely eliminate the warning on returning from a `noreturn` function

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

`DEBUG_ENABLED` should be enabled during debugging, and disabled
afterwards.

sched_switch can check ISR itself.

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.