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