By default (on POSIX) we would be inheriting thread priorities from the
thread that pushed a new task on non-exclusive thread pools and causes a
new thread to be created. This can cause any non-exclusive thread pool
to accidentally contain threads of different priorities, or e.g. threads
with real-time priority.

To prevent this, custom handling for setting the scheduler settings for
Linux and Windows is added and as a fallback for other platforms a new
thread is added that is responsible for spawning threads for
non-exclusive thread pools.

Fixes GNOME/glib#1834