[NTOS]: Fix up POOL_PREV_BLOCK based on suggestion by hpoussin.

[NTOS]: Fix up POOL_PREV_BLOCK based on suggestion by hpoussin.
[NTOS]: Fix up NTAPI location in function definition.
[NTOS]: Implement even more stringent header checks: ExpCheckPoolHeader and ExpCheckPoolBlocks. Normally we would only want this on a DBG build, but I am enabling them for now until I can fix paged pool. If your machine crashes, reverting this commit is NOT the solution (boots for me).
[NTOS]: Add a AllowPagedPool BOOLEAN that will allow us to selectively enable when the ARM3 pool can be used, playing around with the situation that causes the corruption, and perhaps making it easier to find/fix.

svn path=/trunk/; revision=47600