guix system: Load all services on reconfigure, not just stopped ones.

guix system: Load all services on reconfigure, not just stopped ones.

This uses the 'replacement' service slot introduced in the Shepherd
version 0.5.0.

* gnu/services/shepherd.scm (shepherd-service-upgrade): Return a list of
  services that need to be restarted to complete their upgrade.
* guix/scripts/system.scm (call-with-service-upgrade-info): Rename an internal
  variable to reflect the change to shepherd-service-upgrade.
  (upgrade-shepherd-services): Call 'load-services/safe' instead of
  'load-services'.  Print a message about services that need to be
  manually restarted.
* gnu/services/herd.scm (load-services/safe): New procedure.
* doc/guix.texi (Invoking guix system): Document the new behaviour.

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