system: Add 'essential-services' field to <operating-system>.

system: Add 'essential-services' field to <operating-system>.

* gnu/system.scm (<operating-system>)[essential-services]: New field.
(operating-system-directory-base-entries): Remove #:container? keyword
and keep only the not-container branch.
(essential-services): Likewise.
(operating-system-services): Likewise, and call
'operating-system-essential-services' instead of 'essential-services'.
(operating-system-activation-script): Remove #:container?.
(operating-system-boot-script): Likewise.
(operating-system-derivation): Likewise.
* gnu/system/linux-container.scm (container-essential-services): New procedure.
(containerized-operating-system): Use it and set the
'essential-services' field.
(container-script): Remove call to 'operating-system-derivation'.
* gnu/system/vm.scm (system-docker-image): Likewise.
* doc/guix.texi (operating-system Reference): Document 'essential-services'.