pgsql: Add "LP_DEAD item?" column to GiST pageinspect functions

Add "LP_DEAD item?" column to GiST pageinspect functions

This brings gist_page_items() and gist_page_items_bytea() in line with
nbtree's bt_page_items() function.

Minor follow-up to commit 756ab291, which added the GiST functions.

Author: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/E0794687-7315-4C29-A9C7-EC54D448596D@yandex-team.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9e596b65f430fcb942685b41860b323398a88873

Modified Files
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contrib/pageinspect/expected/gist.out         | 32 ++++++++++++------------
contrib/pageinspect/gistfuncs.c               | 18 ++++++++------
contrib/pageinspect/pageinspect--1.8--1.9.sql |  2 ++
doc/src/sgml/pageinspect.sgml                 | 36 +++++++++++++--------------
4 files changed, 46 insertions(+), 42 deletions(-)