Fix thread priority handling

Fix thread priority handling

So far pthread::postcreate() only sets the thread priority at all, only
if the inherit-scheduler attribute is PTHREAD_EXPLICIT_SCHED.  This
completely ignores the PTHREAD_INHERIT_SCHED case, since in contrast
to POSIX, a thread does not inherit its priority from the creating
thread, but always starts with THREAD_PRIORITY_NORMAL.

pthread_getschedparam() only returns what's stored in the thread attributes,
not the actual thread priority.

This patch fixes both problems.

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