Export a few krb5 routines for rxgk

Export a few krb5 routines for rxgk

We need oafs_h_krb5_generate_random_block when generating random
keys and oafs_h_krb5_crypto_fx_cf2 for CombineTokens.
Having oafs_h_krb5_crypto_prf_length proves very convenient for
key derivation of transport keys, so move it to the public header
and export it.
oafs_h_krb5_enctype_keysize is needed so that we can tell whether or not we
need to pass through random_to_key() when making rxgk_keys.
oafs_h_krb5_random_to_key is needed for that random_to_key() operation.

Reviewed-on: https://gerrit.openafs.org/10936
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 348dc87bb2eeb66d1e683dc91ee36724ee18f1af)

Change-Id: I3ae0f5ba6b0bd2f851f4d41f756521153e303102
Reviewed-on: https://gerrit.openafs.org/13104
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>