Fixups for page id field in the archive table:

Fixups for page id field in the archive table:
* Make the field nullable for consistency with ar_rev_id and ar_text_id and between MySQL and PostgreSQL and generally so you can cleanly tell between bogus fields and ones that predate use of the field
* Remove duplicate incorrect adding of the field for postgres
* Rename to ar_page_id for consistency with ar_rev_id and ar_text_id
* Rename the patch file for naming consistency

Note that there's no documentation of this schema change in the release notes either, which bugs me a bit.