gpg: Switch to AES256 for symmetric encryption in de-vs mode.

* g10/gpg.c (set_compliance_option): For AES256 and SHA256 in de-vs
mode.
* g10/encrypt.c (setup_symkey): Add extra compliance check.
(encrypt_simple): Avoid printing a second error oncplinace failure.
--

Because we used the RFC4880 mode as base for the de-vs mode we got
3DES as symmetric encryption algorithm.  With the default gnupg mode
that was already used.  The new extra compliance checks are added to
detect whether a --personal-cipher-preference or --cipher-algo option
tried to override the algorithms.  They are still possible but now
non-compliant algorithms will throw an error.

Manual testing can be done with commands like this:

  gpg --no-options --compliance=de-vs \
   --personal-cipher-preferences "S1 S7" \
   --pinentry-mode loopback -v --passphrase abc -ac </etc/motd

Here the command fails due to IDEA (S1) being the preferred cipher
algorithm.  Using "--s2k-digest-algo SHA1" instead of
--personal-cipher-preferences will also fail.

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit d1f2a6d9f7)

g10/gpg.c

@@ -2188,7 +2188,9 @@ set_compliance_option (enum cmd_and_opt_values option)
     case oDE_VS:
       set_compliance_option (oOpenPGP);
       opt.compliance = CO_DE_VS;
-      /* Fixme: Change other options.  */
+      /* We divert here from the backward compatible rfc4880 algos.  */
+      opt.s2k_digest_algo = DIGEST_ALGO_SHA256;
+      opt.s2k_cipher_algo = CIPHER_ALGO_AES256;
       break;
 
     default: