Fix path reference when parsing pg_ident.conf for pg_ident_file_mappings

Fix path reference when parsing pg_ident.conf for pg_ident_file_mappings

Since a2c8499, HbaFileName (default pg_hba.conf) was getting used
instead of IdentFileName (default pg_ident.conf) as the parent file to
use as reference when parsing the contents of pg_ident.conf, with
pg_ident.conf correctly opened, when feeding this information to
pg_ident_file_mappings.  This had two consequences:
- On an I/O error when reading pg_ident.conf, the user would get an
ERROR message referring to pg_hba.conf and not pg_ident.conf.
- When reading an external file with a relative path using '@' in
pg_ident.conf, the directory used to look at the file to load would be
the base directory of pg_hba.conf rather than the one of pg_ident.conf,
leading to errors in pg_ident_file_mappings inconsistent with what gets
loaded at startup when pg_ident.conf and pg_hba.conf are located in
different directories.

This error only impacted the SQL view pg_ident_file_mappings that uses a
logic new to v15 to fill the view with the parsed information, not the
code paths loading these authentication files at startup.

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20220726050402.vsr6fmz7rsgpmdz3@jrouhaud
Backpatch-through: 15