Replace PG_AUTOCONF_FILENAME with parameter

As opposed to config_file, hba_file, etc. the name and location of the autoconfig file is fixed to $PGDATA/postgresql.auto.conf (guc.h)

The static name also appears in some other locations, such as pg_basebackup, pg_rewind & initdb.c.

New feature suggestion: Allow to set the file's location, as with hba_file and ident_file.

Would be glad to make it my contribution to the code.