Add support for server-side prepared statements in MySQLi backend

The MySQLi backend could support server-side prepared statements (MySQLi statements).. Note that this should not be the default since the client-side prepared statements from Anewt provide features that cannot be used in server-side prepared statements (e.g. ?table? and ?column? stuff, as well as the more exotic ?raw? hackery that is sometimes very useful). Perhaps an API addition like $connection->prepare_server_side() is a good solution.