Explicit config patch 7.2B4

I guess I will maintain this for people who want it.

This allows

postmaster -C /etc/pgsql/mydb.conf

The "-C" option specifies a configuration file.

In the config file there are two more options:

datadir = '/u01/postgres'
hbaconfig = '/etc/pgsql/pg_hba.conf'

The "datadir" option specifies where the postgresql data directory resides. (My
original patch used the setting "pgdatadir" in which the "pg" seemed
redundant.)

The "hbaconfig" specifies where postgresql will look for the pg_hba.conf file.

If the "-D" option is specified on the command line, it overides the "datadir"
option in the config file. (This is a different behavior than my original
patch)

If No "datadir" is specified, it must be specified either on the command line
or the normal PGDATA environment variable.

If no "hbaconfig" setting is set, the it will look for pg_hba.config in the
data directory as always.

One can start many databases with the same settings as:

postmaster -C /path/default.conf -p 5432 -D /path/name1
postmaster -C /path/default.conf -p 5433 -D /path/name2
postmaster -C /path/default.conf -p 5434 -D /path/name3