Expose control file data via SQL accessible functions.

Joe Conway <mail@joeconway.com>

Commit: dc7d70ea05deca9dfc6a25043d406b57cc8f6c30
Author: Joe Conway <mail@joeconway.com>
Date: 2016-03-05T19:10:19Z
Releases: 9.6.0
Expose control file data via SQL accessible functions.

Add four new SQL accessible functions: pg_control_system(),
pg_control_checkpoint(), pg_control_recovery(), and pg_control_init()
which expose a subset of the control file data.

Along the way move the code to read and validate the control file to
src/common, where it can be shared by the new backend functions
and the original pg_controldata frontend program.

Patch by me, significant input, testing, and review by Michael Paquier.

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