Use correct value for anonymous flags

The KDC Option and Ticket Flag for the anonymous extension were changed
from 14 to 16 due to a conflict with S4U2Proxy in version 11 of the anonymous
draft (now RFC6112). Fix the definitions
Recognize anonymous AS requests using bit 14

Check KDC Option bit 14 in addition to 16 when identifying anonymous
AS-REQs. This provides compatibility with older heimdal releases.
Include empty PKINIT-KX padata

rfc6112 requires kdcs implementing anonymous PKINIT to include an
empty PKINIT-KX padata in PREAUTH_REQUIRED messages.
Including this improves compatibility with MIT kerberos.
When using PKINIT with DH, compute session key

RFC6112 provides a method of computing a session key when the PKINIT DH
is used, and mandates it for anonymous pkinit. The session key is computed
using KRB-FX-CF2 from the reply key and a random key chosen by the kdc.
The random key is provided to the client, which is supposed to verify
that the session key was computed this way.
Document logic in _krb5_is_anon_request

describe why we look at the different bits and fields
Use anon realm for anonymous PKINIT

When an AS request names the anonymous principal, use the anonymous
realm in the response and ticket.
Merge pull request #110 from cg2v/anonymous-pkinit

Anonymous pkinit improvements