Possibility to use NOT LIKE expression

Hi,

When i look at this:
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/db/driver/driver.php#L268

I see it's (currently) not possible to use "NOT LIKE" statement.
I'm thinking to add a second parameter which allow us to use NOT LIKE instead of LIKE.

The result should look something like:

$this->_sql_like_expression(($not_like ? 'NOT LIKE' : 'LIKE' ) . ' \'' . $this->sql_escape($expression) . '\'');

I'll create a PR asap.