pgsql: Move BRIN page type to page's last two bytes

Move BRIN page type to page's last two bytes

... which is the usual convention among AMs, so that pg_filedump and
similar utilities can tell apart pages of different AMs. It was also
the intent of the original code, but I failed to realize that alignment
considerations would move the whole thing to the previous-to-last word
in the page.

The new definition of the associated macro makes surrounding code a bit
leaner, too.

Per note from Heikki at
http://www.postgresql.org/message-id/546A16EF.9070005@vmware.com

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/e491bd2ee34860b14ff18abc5602f9aa5b197a2d

Modified Files
--------------
contrib/pageinspect/brinfuncs.c | 13 ++++-------
src/backend/access/brin/brin_pageops.c | 23 +++++--------------
src/backend/access/brin/brin_revmap.c | 2 +-
src/include/access/brin_page.h | 38 ++++++++++++++++++++++++--------
src/include/access/gin_private.h | 8 +++----
src/include/access/spgist_private.h | 2 ++
6 files changed, 45 insertions(+), 41 deletions(-)