GThreadPool: Always use the thread-spawning thread for the global shared thread pool
Setting the main thread's scheduler settings is not reliably possible, especially not if SELinux or similar mechanisms are used to limit what can be done.
As such, get rid of all the complicated code that tried to do this better and use a separate thread for spawning threads for the global shared thread pool. These will always inherit the priority of the main thread (or rather the thread that created the first shared thread pool).
Fixes #2769 (closed)