krb5: Add an optional encrypt_iov function to encryption types

Add a encrypt_iov function pointer to all of our encryption types
which can be used to implement an iovec based encryption routine.

Modify krb5_encrypt_iov so that it calls the iovec based routine
if it is available.

lib/krb5/crypto-arcfour.c

@@ -362,6 +362,7 @@ struct _krb5_encryption_type _krb5_enctype_arcfour_hmac_md5 = {
     &_krb5_checksum_hmac_md5,
     F_SPECIAL | F_WEAK,
     ARCFOUR_encrypt,
+    NULL,
     0,
     ARCFOUR_prf
 };