guix system: 'reconfigure' loads and starts new services.

Partly fixes <http://bugs.gnu.org/22039>.

* gnu/services/herd.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
* gnu/services/shepherd.scm (shepherd-service-canonical-name): New
procedure.
(shepherd-service-file): Export.
* guix/scripts/system.scm (upgrade-shepherd-services): New procedure.
(switch-to-system): Use it.
* guix/ui.scm (info): New procedure.
* doc/guix.texi (Invoking guix system): Mention system services.