Don't allow code to access the PFN database directly -- instead, always go through...

Don't allow code to access the PFN database directly -- instead, always go through MiGetPfnEntry to have a controlled path.
Add assertions to this function, to make sure PFN access is always valid (previous code would sometimes KEBUGCHECK(0) without a real explenation if such cases were encounted -- but developers randomly chose which functions would be protected).
Also, since PFNs are 0-based, do allow Pfn == MmPageArraySize if someone is reading the last PFN on the system.
Finally, protect some of the functions which were accessing the PFN entries outside the PFN list lock.

svn path=/trunk/; revision=32365