* agent/command-ssh.c (struct ssh_key_type_spec): Add field
alt_curve_name.
(ssh_key_types): Add some alternate curve names.
(ssh_identifier_from_curve_name): Lookup also bey alternative names
and return the canonical name.
(ssh_key_to_blob): Simplify the ECDSA case by using gcry_pk_get_curve
instead of the explicit mapping.
(ssh_receive_key): Likewise.  Use ssh_identifier_from_curve_name to
validate the curve name.  Remove the reverse mapping because since
GnuPG-2.2 Libgcrypt 1.7 is required.
(ssh_handler_request_identities): Log an error message.