* gnu/services.scm (system-derivation): New procedure.
(system-service-type): New variable.
(boot-script-entry): New procedure.
(boot-service-type): Extend SYSTEM-SERVICE-TYPE.
(etc-entry): New procedure.
(etc-service-type): Extend SYSTEM-SERVICE-TYPE.
(fold-services): Change default #:target-type to SYSTEM-SERVICE-TYPE.
* gnu/system.scm (operating-system-directory-base-entries): New procedure.
(essential-services): Use it. Add an instance of
SYSTEM-SERVICE-TYPE.
(operating-system-boot-script): Pass #:target-type to 'fold-services'.
(operating-system-derivation): Rewrite in terms of 'fold-services'.
* gnu/system/linux-container.scm (system-container): Remove.
(container-script): Use 'operating-system-derivation'.
* guix/scripts/system.scm (export-extension-graph): Replace
BOOT-SERVICE-TYPE by SYSTEM-SERVICE-TYPE.
* doc/images/service-graph.dot: Add 'system' node and edges.
* doc/guix.texi (Service Composition): Mention SYSTEM-SERVICE-TYPE.
(Service Reference): Document it. Update 'fold-services'
documentation.
