The following patch makes postmaster -D work. -D specifies a different PGDATA

The following patch makes postmaster -D work.  -D specifies a different PGDATA
directory.  The code that looks for the pg_hba file doesn't use it, though,
so the postmaster uses the wrong pg_hba file.  Also, when the postmaster
looks in one directory and the user thinks it is looking in another
directory, the error messages don't give enough information to solve the
problem.  I extended the error message for this.

Submitted by: Bryan Henderson <bryanh@giraffe.netgate.net>