[Pana] pana-preauth-06 available

[Yoshihiro Ohba <yoshihiro.ohba at toshiba.co.jp>]

Changes:

- Revised definition of "Serving Network".

- Added an informational reference for 802.21 as an example mechanism 
for discovering PaC and CPAA to discover each other.

Basavaraj questioned about the need for PAA-initiated pre-auth, and I 
agreed with his comment. But
PAA-initiated pre-auth still remains, based on Alper's comment "PANA 
already supports PAA-initiated auth. So, unless we do anything special 
(to eliminate this case), it comes for free."

Best Regards,
Yoshihiro Ohba