datestyle=postgres broken with timezone=UTC+N

Discovered when debugging libpqtypes test failures:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=795729

postgres =# set timezone = 'Etc/UTC+1';
SET
postgres =# set datestyle = 'postgres';
SET
postgres =# select '2015-01-01 01:00:00 +0100'::timestamptz;
Wed 31 Dec 23:00:00 2014 ETC/UTC

postgres =# select 'Wed 31 Dec 23:00:00 2014 ETC/UTC'::timestamptz;
Wed 31 Dec 22:00:00 2014 ETC/UTC

That is, feeding back a value will to the UTC offset shift again.
(9.6 head, also seen on 9.4.)

Christoph
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