agent: Put ssh key type as comment into sshcontrol.
* agent/command-ssh.c (ssh_key_type_spec): Add field name. (ssh_key_types): Add human readable names. (add_control_entry): Add arg SPEC and print key type as comment. (ssh_identity_register): Add arg SPEC. (ssh_handler_add_identity): Add var SPEC and pass ssh_receive_key.