* gnu/services/ssh.scm (<openssh-configuration>)[authorized-keys]: New
field.
(authorized-key-directory): New procedure.
(openssh-config-file): Honor 'authorized-keys'.
(openssh-activation): Use 'with-imported-modules'. Make /etc/ssh
755. Create /etc/ssh/authorized_keys.d.
* doc/guix.texi (Networking Services): Document it.