Integrate recovery.conf into postgresql.conf

recovery.conf settings are now set in postgresql.conf (or other GUC
sources).  Currently, all the affected settings are PGC_POSTMASTER;
this could be refined in the future case by case.

Recovery is now initiated by a file recovery.signal.  Standby mode is
initiated by a file standby.signal.  The standby_mode setting is
gone.  If a recovery.conf file is found, an error is issued.

The trigger_file setting has been renamed to promote_trigger_file as
part of the move.

The documentation chapter "Recovery Configuration" has been integrated
into "Server Configuration".

pg_basebackup -R now appends settings to postgresql.auto.conf and
creates a standby.signal file.

Author: Fujii Masao <masao.fujii@gmail.com>
Author: Simon Riggs <simon@2ndquadrant.com>
Author: Abhijit Menon-Sen <ams@2ndquadrant.com>
Author: Sergei Kornilov <sk@zsrv.org>
Discussion: https://www.postgresql.org/message-id/flat/607741529606767@web3g.yandex.ru/