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.