Usability: display a message after user registration

After user registers (and no confirmation is required), Drupal automatically logs user in without any message. In the means of usability, this is more complex, but for D6, it should be enough to display a message.

This patch adds one line to user_register_submit() which does the thing. It's an usability improvement and i hope it gets accepted to D6.

For D7, the approach should be (Thanks to Czech usability expert, Marek Prokop):
1) Register
2) Drupal automatically logs in
3) Redirect to special page (user/register-information) which is static, displays all required information about user registration (username, email, how to log in, how to recover password, what to do now). The advantage is that this straightforwards the workflow and allows user to print/save neccessary information.