Unify the iop-subcommands install_key and ssh_install_key to one script.
In addition:
* Support new key types such as ssh-ed25519 which is supported by
Dropbear nowadays. Also add unsupported ones to make them work
automatically in the future.
* For the install_key command, do not re-add keys already added
* Read keys loaded into SSH Agent as well, which is useful when working
with agent forwarding on a remote host or in a Docker container.
* For the ssh_install_key command also add keys added manually to the
build dir.
