Unset System User Password

There was a limitation in the way the system user (typically 'postgres') is handled with respect to passwords.  You could change the password to any non-empty value and it would be honored.  However, if you tried to unset the password (make it NULL) the code would either ignore the request or create an unusable hashed password.

This code fixes that problem by allowing the directive 'postgres_password => false' to be specified, which will set the user password to NULL in the database.

This is a partial fix for ticket #254.  Assuming this methodology is considered acceptable, we can create a subsequent patch generalized for normal user accounts using this same scheme.