gpg: Allow only OCB for AEAD encryption.
* g10/gpg.c (opts): New option--force-ocb as alias for force-aead. Turn --aead-algo and --personal-aead-preferences into dummy options. (build_list_md_test_algo, build_list_aead_algo_name): Remove. (my_strusage): Remove output of AEAD algos. (main): Remove code from the --aead options. * g10/encrypt.c (encrypt_seskey): Make file local. (use_aead): Remove requirement for rfc4880bis. Always return AEAD_ALGO_OCB. * g10/main.h (DEFAULT_AEAD_ALGO): Removed unused macro. * g10/misc.c (default_aead_algo): Remove. * g10/pkclist.c (select_aead_from_pklist): Return AEAD_ALGO_OCB or 0. (select_algo_from_prefs): Remove personal AEAD algo setting. * g10/keygen.c (keygen_set_std_prefs): Remove AEAD preference option parsing. * g10/options.h (opt): Remove def_aead_algo and personal_aead_prefs.
Due to the meanwhile expired patent on OCB there is no more reason for
using EAX. Thus we forcefully use OCB if the AEAD feature flag is set
on a key.