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.

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