machine: ssh: Check for potential system downgrades.

This is a followup to 8e31736b.

* guix/scripts/system/reconfigure.scm (check-forward-update): Add
 #:current-channels.  Use it instead of OLD.
* gnu/services.scm (sexp->system-provenance): New procedure.
(system-provenance): Use it.
* gnu/machine/ssh.scm (<machine-ssh-configuration>)[allow-downgrades?]:
New field.
(machine-check-forward-update): New procedure.
(check-deployment-sanity)[assertions]: Call it.
* doc/guix.texi (Invoking guix deploy): Document 'allow-downgrades?'
field.