Add a new 'proxy' field to openssh-host to allow ProxyCommand or
ProxyJump, but not both, to be configured. Configuring both would cause
the serialization order to determine which one is used. Deprecate the
'proxy-command' field because the 'proxy' field replaces it.
* gnu/home/services/ssh.scm (proxy-jump->string,
proxy-command-or-jump-list?, serialize-proxy-command-or-jump-list,
sanitize-proxy-command): New procedure.
(proxy-jump, proxy-command): New record type.
(openssh-host)[proxy-command]: Mark field as deprecated because OpenSSH
can't have ProxyCommand and ProxyJump configured at the same time.
* doc/guix.texi (Secure Shell): Update to match the changes to the
service.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>