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 GNOME/glib#2769