gpg: Print info about the used AEAD algorithm in the compliance msg.

* g10/misc.c (openpgp_cipher_algo_mode_name): New.
* g10/decrypt-data.c (decrypt_data): Use function here.

Note that openpgp_cipher_algo_mode_name is different from the version
2.2 becuase we append ".CFB" here.

Without this change we would see

gpg: cipher algorithm 'AES256' may not be used in
--compliance=de-vs mode

This is confusing because AES256 is compliant. Now we see

gpg: cipher algorithm 'AES256.OCB' may not be used in
--compliance=de-vs mode

which gives a hint on the problem.