uc_signup_order_prepare() ignores a user's user name preference

Issue Summary

Ubercart handles new account creation by preferring a user's user name preference and then using uc_store_email_to_username(). Ubercart signup only uses the latter so the user name form field becomes useless.

Either uc_signup should hide the user name form field from the user registration process on signup, or should grab the form value somehow. I am not familiar with how uc_signup fits into the Ubercart process so the latter option may or may not be possible.