Add an AEAD provider for enctypes which use krb5_old_encrypt and

Add an AEAD provider for enctypes which use krb5_old_encrypt and
krb5_old_decrypt; this makes every enctype have an AEAD provider.  To
make this work, expose make_unkeyed_checksum_iov to other files (under
the name krb5int_hash_iov) and make krb5int_c_padding_length take into
account the header length.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23388 dc483132-0cff-0310-8789-dd5450dbe970