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-aes-sha1.c

@@ -154,6 +154,7 @@ struct _krb5_encryption_type _krb5_enctype_aes128_cts_hmac_sha1 = {
     &_krb5_checksum_hmac_sha1_aes128,
     F_DERIVED | F_RFC3961_ENC | F_RFC3961_KDF,
     _krb5_evp_encrypt_cts,
+    NULL,
     16,
     AES_SHA1_PRF
 };
@@ -170,6 +171,7 @@ struct _krb5_encryption_type _krb5_enctype_aes256_cts_hmac_sha1 = {
     &_krb5_checksum_hmac_sha1_aes256,
     F_DERIVED | F_RFC3961_ENC | F_RFC3961_KDF,
     _krb5_evp_encrypt_cts,
+    NULL,
     16,
     AES_SHA1_PRF
 };