Implement sceKernelChangeThreadPriority() and sched fixes

Please accept hrydgard/pspautotests#79 first.

May help Gran Turismo:
http://forums.ppsspp.org/showthread.php?tid=1601&pid=26924#pid26924

Has some interesting functionality, seems to work to duplicate the current thread's priority onto another, and also to yield.

Also adds checks to sceKernelChangeCurrentThreadAttr() and fixes the priority of a thread that is restarted (which has interesting logic...)  Tried a number of games that call these, and they all seem happy.

-[Unknown]