system, home: Validate ‘services’ field value....This guides newcomers who might stick a single (service …) form
in this field.
* gnu/services.scm (validate-service-list): New macro.
(%validate-service-list): New procedure.
* gnu/system.scm (<operating-system>)[services]: Add ‘sanitize’.
* gnu/home.scm (<home-environment>)[services]: Add ‘sanitize’.
Change-Id: I9e29bd9a078e87b627ab766fd669ba9de79f8473