services: Add 'openssh-service'.

* gnu/packages/ssh.scm (openssh)[arguments]: Set sysconfdir to /etc/ssh.
* gnu/services/ssh.scm (<openssh-configuration>): New record type.
(%openssh-accounts): New variable.
(openssh-activation, openssh-config-file, openssh-shepherd-service)
(openssh-service): New procedures.
(openssh-service-type): New variable.
* doc/guix.texi (Networking Services): Document 'openssh-services'.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>