Derive DES/fcrypt session key from other key types

Derive DES/fcrypt session key from other key types

If a kerberos 5 ticket has a session key with a non-DES enctype,
use the NIST SP800-108 KDF in counter mode with HMAC_MD5 as the PRF to
construct a DES key to be used by rxkad.

To satisfy the requirements of the KDF, DES3 keys are first compressed into a
168 bit form by reversing the RFC3961 random-to-key algorithm

Change-Id: I4dc8e83a641f9892b31c109fb9025251de3dcb27