Explicit configuration file

I have added the option of explicitly specifying a postgresql.conf (-C) file on
the command line. I have also added two config file entries:

pghbaconfig, and pgdatadir.

"pghbaconfig" allows multiple different databases running on the same machine
to use the same hba file, rather than the hard coded $PGDATA/pg_hba.conf.

"pgdatadir" works with the explicit configuration file so that the data
directory can be specified in the configuration file, not on the command line
or environment.

One can start postgres as:

postmaster -C /etc/pgsql/mydb1.conf

Postgres will get all its required information from "mydb1.conf."

Does anyone see any value to these mods? I wanted to be able to run multiple
PostgreSQL instances on the same machine, and having the ability to keep these
control files in a central location and share the HBA control files between
databases may be helpful for admins. It will certainly make my life easier.