Add option --config-file to pg_rewind

Add option --config-file to pg_rewind

This option is useful to do a rewind with the server configuration file
(aka postgresql.conf) located outside the data directory, which is
something that some Linux distributions and some HA tools like to rely
on.  As a result, this can simplify the logic around a rewind by
avoiding the copy of such files before running pg_rewind.

This option affects pg_rewind when it internally starts the target
cluster with some "postgres" commands, adding -c config_file=FILE to the
command strings generated, when:
- retrieving a restore_command using a "postgres -C" command for
-c/--restore-target-wal.
- forcing crash recovery once to get the cluster into a clean shutdown
state.

Author: Gunnar "Nick" Bluth
Reviewed-by: Michael Banck, Alexander Kukushkin, Michael Paquier,
Alexander Alekseev
Discussion: https://postgr.es/m/7c59265d-ac50-b0aa-ca1e-65e8bd27642a@pro-open.de