Implement function to drop priority of current thread

Implement function to drop priority of current thread
This function is useful for libraries like ecore and evas that have to
set some worker threads. The first thing these threads should do is to
call this function, so the main thread might continue running without
the worker threads interrupting it too much.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/eina@52651 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33