Add missing KRB-FX-CF2 test vectors (3DES broken)

Note that this shows that the our KRB-FX-CF2 is broken as to 3DES (and
the 1DES PRF is still missing).
Fix DES3 PRF

RFC 3961 says the simplified profile PRF should truncate the hash
output to "multiple of m", which MIT krb5 interprets as the largest
possible multiple of m.  RFC 6113 appendix A also uses that
interpretation for the KRB-FX-CF2 test vector.  So the DES3 PRF should
truncate the 20-byte SHA-1 result to 16 bytes, not 8.  Also make
krb5_crypto_prf_length work with DES3 by giving the DES3 enctype a
non-zero PRF length.

Signed-off-by: Nicolas Williams <nico@cryptonector.com>
Fix KRB-FX-CF2 for enctypes with non-dense keyspaces

It is necessary to use the RFC3961 random_to_key operation when
creating a key from a bitstring.

Signed-off-by: Nicolas Williams <nico@cryptonector.com>