agent: Support the Ed25519 signature algorithm for ssh.
* agent/command-ssh.c (SPEC_FLAG_IS_EdDSA): New. (ssh_key_types): Add entry for ssh-ed25519. (ssh_identifier_from_curve_name): Move to the top. (stream_read_skip): New. (stream_read_blob): New. (ssh_signature_encoder_rsa): Replace MPIS array by an s-exp and move the s-exp parsing to here. (ssh_signature_encoder_dsa): Ditto. (ssh_signature_encoder_ecdsa): Ditto. (ssh_signature_encoder_eddsa): New. (sexp_key_construct): Rewrite. (ssh_key_extract): Rename to ... (ssh_key_to_blob): .. this and rewrite most of it. (ssh_receive_key): Add case for EdDSA. (ssh_convert_key_to_blob, key_secret_to_public): Remove. (ssh_send_key_public): Rewrite. (ssh_handler_request_identities): Simplify. (data_sign): Add rename args. Add new args HASH and HASHLEN. Make use of es_fopenmen and es_fclose_snatch. Remove parsing into MPIs which is now doe in the sgnature encoder functions. (ssh_handler_sign_request): Take care of Ed25519. (ssh_key_extract_comment): Rewrite using gcry_sexp_nth_string.
To make the code easier readable most of the Ed25591 work has been
done using a new explicit code path. Warning: Libgcrypt 1.6.1 uses a
non optimized implementation for Ed25519 and timing attacks might be
possible.
While working on the code I realized that it could need more rework;
it is at some places quite baroque and more complicated than needed.
Given that we require Libgcrypt 1.6 anyway, we should make more use of
modern Libgcrypt functions.