Gpgsql backend config

Change defaults for gpgsql-dbname and gpgsql-user to empty strings (postgres is using the username of the system user the process is running as for user and dbname); only append dbname=/user= to the connect string if the value is not empty.

If host is empty postgres uses unix sockets to connect to the database server (if available on the platform); the default authentication method on unix sockets is "ident" using the system user of the connecting process, so a hardcoded "powerdns" user is a bad idea, especially as postgres has a better default.