Catalog version access

Hello.

I do some very regular testing on HEAD and my scripts need to know if
the catalog version has changed to determine if it needs to pg_restore
or if a basebackup is okay. In order to get it, I have to do this:

# Get the catalog version (there is no better way to do this)
tmp=$(mktemp --directory)
$bin/initdb --pgdata=$tmp
catversion=$($bin/pg_controldata $tmp | grep "Catalog version" \
| cut --delimiter=: --fields=2 | xargs)
rm --recursive --force $tmp

I find this less than attractive, especially since the catalog version
is a property of the binaries and not the data directory. Attached is a
patchset so that the above can become simply:

catversion=$($bin/pg_config --catversion)

and a second patch that adds a read-only guc to get at it on the SQL
level using SHOW catalog_version; or similar. I need that because I
also do a dump of pg_settings and I would like for it to appear there.

Please consider.
--
Vik Fearing