Add osm2pgsql flat node support via settings.php

New configuration option CONST_Osm2pgsql_Flatnode_File will cause Nominatim to set the --flat-nodes option on all calls to osm2pgsql. The value of the option must be a full path name accessible to the user running setup.php/update.php, ideally pointing to SSD storage. The flatnode option Improves performance and reduces disk usage for full planet operation.