Save the settings in a database table

Considering #668708: Provide settings for each search page, it is better to save the settings in a database table, rather than in Drupal variables.

It is inefficient to call variable_get() for each variable used when it would be possible to get all the settings with a single db_query() call, and it doesn't allow to easily add more settings; it would be also a little more difficult to do it when the code will be changed as per #668708: Provide settings for each search page.