system: Automatically add essential services.
* gnu/services/base.scm (%base-services): Remove calls to
'host-name-service', 'user-processes-service', and
'root-file-system-service'.
* gnu/system.scm (<operating-system>)[operating-system-services]: Rename
to...
[operating-system-user-services]: ... this.
(essential-services, operating-system-services): New procedures.
(operating-system-accounts, operating-system-etc-directory,
operating-system-boot-script, operating-system-derivation): Adjust to
new 'operating-system-services' return type.