system: Define 'device-mapping-kind', and add a 'close' procedure.

* gnu/system/file-systems.scm (<mapped-device-type>): New record type.
  (<mapped-device>)[command]: Remove field.
  [type]: New field.
* gnu/services/base.scm (device-mapping-service): Rename 'command'
  parameter to 'open'.  Add 'close' parameter and honor it.
* gnu/system.scm (luks-device-mapping): Rename to...
  (open-luks-device): ... this.
  (close-luks-device): New procedure.
  (luks-device-mapping): New variable.
  (device-mapping-services): Get the type of MD, and pass its 'open' and
  'close' fields to 'device-mapping-service'.