Improve QlResponse, add QlNullResponseEntity and QlStaticResponseEntity

Add QlResponse::send_file(), rename QlResponse::allow_caching() to
use_cache() so that it looks more intuitive when called
(“if (…->use_cache(…))”), and fix a few minor issues in its code flow.

Enable implementations of QlModule::handle_static_request(), which now can
return QlStaticResponseEntity or QlNullResponseEntity depending on the
remote client’s caching of the entity.

Remove “exit;” statements, aiming to completely get rid of them in the
entire source tree.