Add a compatibility with sharedemail module

sharedemail: The shared email module overrides the 'user' module's validation that prevents the same email address being used by more than one user. Works for both registration and account updates. Displays a warning to the user that they are using a shared email.

Really it adds a prefix 'sharedemail_' to $edit['mail'] on hook_user($op = 'validate') preventing the core from generating a warning. Later this module roll's back the user's mail to it's original value.

Of course this changed email doesn't pass email verification, so we can just check for this 'sharedemail_' prefix and remove it if needed.