system: Add support for Linux-style mapped devices.
* gnu/system/file-systems.scm (<mapped-device>): New record type.
* gnu/system.scm (<operating-system>)[mapped-devices]: New field.
(luks-device-mapping): New procedure.
(other-file-system-services)[device-mappings, requirements]: New
procedures. Pass #:requirements to 'file-system-service'.
(device-mapping-services): New procedure.
(essential-services): Use it. Append its result to the return value.
(operating-system-initrd-file): Add comment.
* gnu/services/base.scm (file-system-service): Add #:requirements
parameter and honor it.
(device-mapping-service): New procedure.
* gnu/system/linux-initrd.scm (base-initrd): Add comment.