This allows squeezing out the unused space in full-page writes. And more
importantly, it can be a useful debugging aid.

In hindsight we should've done this back when GIN was added - we wouldn't
need the 'maxoff' field in the page opaque struct if we had used pd_lower
and pd_upper like on normal pages. But as long as there can be pages in the
index that have been binary-upgraded from pre-9.4 versions, we can't rely
on that, and have to continue using 'maxoff'.

Most of the code churn comes from renaming some macros, now that they're
used on internal pages, too.

This change is completely backwards-compatible, no effect on pg_upgrade.