Configuration file patch

This patch allows using PostgreSQL with a command line configuration
file parameter instead of the data directory.

If no configuration is specified, postmaster operates as it always has.

The configuration file is specified with the "-C" parameter, as:

postmaster -C /somepath/somefile.conf

Within the configuration file, there are three new parameters: data_dir,
hba_conf, and ident_conf. They are used as:

hba_conf = 'pathanme_to_pg_hba.conf'
ident_conf='pathname_to_pg_ident.conf'
data_dir='path_to_data'

If the above parameters are not specified, then the default is to look
for these file in the PGDATA directory.
Command line arguments take precedent over configuration file.

This patch is not a be-all end-all of configuration. It should be able
to fit PostgreSQL into a FHS with the exception of the '/var/run'
requirement.