Anonymous pkinit improvements

This patchset improves the heimdal kdc's compatibility with MIT kerberos when anonymous pkinit is used:

- use bit 16 for anonymous kdc option and ticket flag instead of 14 (partial support for heimdal clients that 
use bit 14 for anonymous is included. No partial support for heimdal clients that use bit 16 for S4U2Proxy)
- send PKINIT-KX pa responses, and compute session key according to rfc6112 when needed
- For (some more) conformance with rfc6112, use anonymous realm when AS client principal is the anonymous principal.