Request: use auth-Hooks for user registering too

auth-Hooks allow only "login"

I 'm missing function-hooks to extend the connection to the external authentication instance to:

• user registering

• validating the parameters
• user_add

following would come in next step, "password changes" transmit to the external auth instance too.

Attached a patch which extends functions_user.php
for external user-registration process

new hook-function possible
user_add_EXTERNAUTH
validate_username_EXTERNAUTH (user already exists?)
validate_email_EXTERNAUTH (email already exists?)

So all authentication and registering is transparent phpbb-like, and the enduser has not to switch the applications.