Signup: if chosen handle is unavailable, password and captcha have to be entered again

Hi,

The sign-up process can become a bit annoying due to the late feedback about handle availability.

Problem here: the handle is not checked for availability immediately. So every time I submit the sign-up data with a handle that is not available, I get an error back and have to re-enter the password as well as the password confirmation, and then check the captcha box again. This can get a bit annoying after going through a couple of iterations. It would be good to have immediate feedback on the chosen handle _before_ submitting the sign-up form. Thanks!