Implemented the "default namespaces for search" feature. This includes

Implemented the "default namespaces for search" feature. This includes
changes to the preferences form, a new key in the LanguageXX.php files
(defaultns) and a new global config variable in DefaultSettings.php
($wgNamespacesToBeSearchedDefault).

I also killed a bug that produced an errorneous SQL command when the user
failed to check any namespaces to search in the search form. The updated
implementation searched the main namespace if no checkbox is checked.