postgresql.auto.conf read from wrong directory

Hi,

if you split configuration and data by setting data_directory,
postgresql.auto.conf is writen to the data directory
(/var/lib/postgresql/9.4/main in Debian), but tried to be read from
the etc directory (/etc/postgresql/9.4/main).

One place to fix it would be in ProcessConfigFile in
src/backend/utils/misc/guc-file.l:162 by always setting
CallingFileName = NULL in src/backend/utils/misc/guc-file.l:162, but
that breaks later in AbsoluteConfigLocation() when data_directory is
NULL. (As the comment in ProcessConfigFile says.)

I'm not sure where to break that dependency loop - the fix itself
seems easy, just don't tell to look for postgresql.auto.conf next to
ConfigFileName, but in the data_directory.

Christoph
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