When sigma0 runs on a lower priority than the rest of the threads in the
system it might come to the point that while answering a page fault or
I/O memory area request the timeslice of the caller (core-pager) gets
fully consumed. As long as other threads are still executable and don't block
sigma0 won't do progress anymore, because it runs at the lowest priority.
This commit simply sets sigma0's priority to the highest in the system.
