gnu: services: Add %hurd-startup-service.

gnu: services: Add %hurd-startup-service.

This decouples startup of the Hurd from the "hurd" package, moving the RC
script into SYSTEM.

* gnu/packages/hurd.scm (hurd)[inputs]: Remove hurd-rc-script.
[arguments]: Do not substitute it.  Update "runsystem.sh" to parse kernel
arguments and exec into --system=SYSTEM/rc.
(hurd-rc-script): Move to...
* gnu/services.scm (%hurd-rc-file): ...this new variable.
(hurd-rc-entry): New procedure.
(%hurd-startup-service): Use it in new variable.
* gnu/system.scm (hurd-default-essential-services): Use it.