Rewrite scheduler functions getting and setting process and thread priority

Rewrite scheduler functions getting and setting process and thread priority

So far the scheduler priority handling is not POSIX compatible.
The priorities use a range of -14 up to +15, which means it's not clear
if the POSIX-required return value of -1 in case of an error is *really*
an error or just the valid priority value -1.  Even more confusing, -14
is the *max* value and 15 is the *min* value.  Last but not least this
range doesn't match the POSIX requirement of at least 32 priority values.

This patch cleans up scheduler priority handling and moves the valid
priority range to 1 (min) - 32 (max).  It also adds a function
sched_get_thread_priority() which will help to make thread priority
more POSIX-like.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>