PATCH: add "--config-file=" option to pg_rewind

Hi!

During a Patroni PR discussion
(https://github.com/zalando/patroni/pull/2225) we realised that if one
wants to use the "-c" option on a typical Debian/Ubuntu installation
(where the config resides below /etc/postgresql/), pg_rewind needs a way
to be told where the postgresql.conf actually is.

The attached patch implements this as an option.

I'm extremely unhappy that I called it "--config-file" here, while
"postgres" itself wants "--config_file". But as the other dashed options
of pg_rewind are, well, dashed and not underscored, it seemed to be
better that the other way...

As the "-c" feature appeared in 12 and existing Debian installations are
unable to use it right now, I suggest to even backport the patch.

Oh, and I'm still not subscribed to -hackers, so the usual "please CC
me" applies!

Best regards,
--
Gunnar "Nick" Bluth

Eimermacherweg 106
D-48159 Münster

Mobil +49 172 8853339
Email: gunnar(dot)bluth(at)pro-open(dot)de
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"Ceterum censeo SystemD esse delendam" - Cato