pgsql: Fix multiple bugs in contrib/pgstattuple's pgstatindex() functio

Fix multiple bugs in contrib/pgstattuple's pgstatindex() function.

Dead or half-dead index leaf pages were incorrectly reported as live, as a
consequence of a code rearrangement I made (during a moment of severe brain
fade, evidently) in commit d287818eb514d431.

The index metapage was not counted in index_size, causing that result to
not agree with the actual index size on-disk.

Index root pages were not counted in internal_pages, which is inconsistent
compared to the case of a root that's also a leaf (one-page index), where
the root would be counted in leaf_pages. Aside from that inconsistency,
this could lead to additional transient discrepancies between the reported
page counts and index_size, since it's possible for pgstatindex's scan to
see zero or multiple pages marked as BTP_ROOT, if the root moves due to
a split during the scan. With these fixes, index_size will always be
exactly one page more than the sum of the displayed page counts.

Also, the index_size result was incorrectly documented as being measured in
pages; it's always been measured in bytes. (While fixing that, I couldn't
resist doing some small additional wordsmithing on the pgstattuple docs.)

Including the metapage causes the reported index_size to not be zero for
an empty index. To preserve the desired property that the pgstattuple
regression test results are platform-independent (ie, BLCKSZ configuration
independent), scale the index_size result in the regression tests.

The documentation issue was reported by Otsuka Kenji, and the inconsistent
root page counting by Peter Geoghegan; the other problems noted by me.
Back-patch to all supported branches, because this has been broken for
a long time.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/0276bbd6234a54c5e646f728131d204068fd0800

Modified Files
--------------
contrib/pgstattuple/expected/pgstattuple.out | 44 +++++++++++++++-------------
contrib/pgstattuple/pgstatindex.c | 16 ++++------
contrib/pgstattuple/sql/pgstattuple.sql | 26 ++++++++++++----
doc/src/sgml/pgstattuple.sgml | 29 +++++++++++-------
4 files changed, 68 insertions(+), 47 deletions(-)