PostgreSQL (drupal_write_record) fix

Hi,

REPLACE INTO does not work on PostgreSQL, and I had just read through other code using drupal_write_record(). (twitter/twitter.inc)
So I copied those lines.

By the way, hook_schema should be named after the module, not after the table. (If you don't rename that function in system_module.install, drupal_get_schema('system_module_users') will return nothing, and drupal_write_record() will do nothing.)