[Serusers] UAC module (backport to 0.9.0)

[Zen Kato]

Can we register to 'asterisk' after registration success to 'ser'
with one 'ser' account using UAC module as follows?

UA REGISTER sip:123 at ser     --> ser
UA Unauthorized            <--  ser
UA REGISTER w/Authorization --> ser
UA 200 OK <-------------------- ser
                                ser -->REGISTER sip:123 at asterisk   *
                                ser<-- Unauthorized                *
                                ser -->REGISTER w/Authorization    *
                                ser<-- 200 OK
UA 200 OK <-------------------- ser

This UA has only one sip account like BT101 or Windows Messenger5.1.

Regards,

Zen