Before that 'open-connection' would return #f, and thus
'current-services' would return a single #f value when its continuation
expects two.
Reported by calher on #guix.
* gnu/services/herd.scm (open-connection): Rethrow system-error
exceptions.
(with-shepherd): Expect CONNECTION to always be true; remove useless
'dynamic-wind'.
* guix/scripts/system.scm (warn-on-system-error): New macro.
(upgrade-shepherd-services): Wrap body in 'warn-on-system-error'.
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.
