Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
These headers do not provide full stl functionality,
but a small subset:
* thread and this_thread
* condition_variable (some timed functions are missing)
* mutex, lock_guard and unique_lock
