pgsql: Add relkind checks to certain contrib modules

Add relkind checks to certain contrib modules

The contrib extensions pageinspect, pg_visibility and pgstattuple only
work against regular relations which have storage. They don't work
against foreign tables, partitioned (parent) tables, views, et al.

Add checks to the user-callable functions to return a useful error
message to the user if they mistakenly pass an invalid relation to a
function which doesn't accept that kind of relation.

In passing, improve some of the existing checks to use ereport() instead
of elog(), add a function to consolidate common checks where
appropriate, and add some regression tests.

Author: Amit Langote, with various changes by me
Reviewed by: Michael Paquier and Corey Huinker
Discussion: https://postgr.es/m/ab91fd9d-4751-ee77-c87b-4dd704c1e59c@lab.ntt.co.jp

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c08d82f38ebf763b79bd43ae34b7310ee47aaacd

Modified Files
--------------
contrib/pageinspect/expected/page.out | 9 ++
contrib/pageinspect/rawpage.c | 5 +
contrib/pageinspect/sql/page.sql | 9 ++
contrib/pg_visibility/.gitignore | 4 +
contrib/pg_visibility/Makefile | 2 +
contrib/pg_visibility/expected/pg_visibility.out | 142 +++++++++++++++++++++++
contrib/pg_visibility/pg_visibility.c | 58 ++++++---
contrib/pg_visibility/sql/pg_visibility.sql | 83 +++++++++++++
contrib/pgstattuple/expected/pgstattuple.out | 107 +++++++++++++++++
contrib/pgstattuple/pgstatindex.c | 52 +++++++--
contrib/pgstattuple/pgstattuple.c | 3 +
contrib/pgstattuple/sql/pgstattuple.sql | 64 ++++++++++
12 files changed, 517 insertions(+), 21 deletions(-)