* common/openpgp-oid.c (oidtable): Add X448.
(oid_x448,openpgp_oidbuf_is_x448,openpgp_oid_is_x448): New.
* common/util.h (openpgp_oid_is_x448): New.
* g10/ecdh.c (gen_k): Add handling of opaque MPI and support
endianness.
(pk_ecdh_generate_ephemeral_key): X448 requires opaque MPI.
* g10/keygen.c (gen_ecc): Add support for X448.
(ask_algo, parse_key_parameter_part): Likewise.