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

doc/src/sgml/ref/pg_rewind.sgml

@@ -241,6 +241,21 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--config-file=<replaceable class="parameter">filename</replaceable></option></term>
+      <listitem>
+       <para>
+        Use the specified main server configuration file for the target
+        cluster. This affects <application>pg_rewind</application> when
+        it uses internally the <application>postgres</application> command
+        for the rewind operation on this cluster (when retrieving
+        <varname>restore_command</varname> with the option
+        <option>-c/--restore-target-wal</option> and when forcing a
+        completion of crash recovery).
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>--debug</option></term>
       <listitem>