Add new system view, pg_config

Joe Conway <mail@joeconway.com>

Commit: a5c43b886942e96ec5c745041f2d6a50c3205147
Author: Joe Conway <mail@joeconway.com>
Date: 2016-02-17T17:12:06Z
Releases: 9.6.0
Add new system view, pg_config

Move and refactor the underlying code for the pg_config client
application to src/common in support of sharing it with a new
system information SRF called pg_config() which makes the same
information available via SQL. Additionally wrap the SRF with a
new system view, as called pg_config.

Patch by me with extensive input and review by Michael Paquier
and additional review by Alvaro Herrera.

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Documentation touched