Add support for TNS names in D7 driver

I don't know if its possible, since Drupal seems to want a hostname during install. But this is what I did (patched database.inc:76) to allow me to use a TNS name instead of a database host/port/instance.

    if ($connection_options['host'] == 'USETNS') {
      // Use database as TNSNAME
      $dsn = 'oci:dbname='.$connection_options['database'] . ';charset=AL32UTF8';
    }
    else {
      // Use host/port/database
      $dsn = 'oci:dbname=//' . $connection_options['host'] . ':' . $connection_options['port'].'/' . $connection_options['database'] . ';charset=AL32UTF8';
    }

You'd also have to update the install.txt to let possible users know how to do this.