Reworked handling of illegal virtual aliases caused by frame reuse.
We moved the incomplete handling from backend's frame method to
backend's page_fault method. The page_fault method is the one that
can create an illegal alias if it writes the userspace frame using
kernel address with a different page color than the page to which is
this frame mapped in userspace. When we detect this, we do D-cache
shootdown on all processors (!!!).
If we add code that accesses userspace memory from kernel address
space, we will have to check for illegal virtual aliases at all such
places.
I tested this on a 4-way simulated E6500 and a real-world Ultra 5,
which has unfortunatelly only one processor.
This solves ticket #26.
