WIP checksums patch

This is just a rebased version of the patch by Simon here:

http://archives.postgresql.org/message-id/CA
+U5nMKw_GBs6qQ_Y8-RjGL1V7MVW2HWBHartB8LoJhnPfxL8g(at)mail(dot)gmail(dot)com

There are other things I'd like to do, like:

* include page# in checksum, and perhaps relfilenode or object OID (but
those two might cause difficulties)
* use TLI field instead of page version, if that is the consensus
(though it seems some may still disagree with that)
* we might want to make it slightly easier for external utilities, like
for backup/replication, to verify the pages
* optionally protect temporary files and local buffers
* come up with a robust way to determine that all pages in the database
are protected
* protect uninitialized pages

But not all of these will make the first patch. This is just a starting
point to reignite the discussion.

I welcome any feedback.

Regards,
Jeff Davis