Validate params in sceKernelChangeThreadPriority().
Reschedule in sceKernelChangeThreadPriority().

Seems like this is another way to yield, makes sense.
sceKernelChangeThreadPriority() can't do dormant.
Reset restarted thread priorities properly.
Reporting in sceKernelChangeCurrentThreadAttr().
Update tests.
Merge pull request #2106 from unknownbrackets/thread-funcs

Correct sceKernelChangeThreadPriority() and sched fixes