services: configuration: Allow disabling serialization.

services: configuration: Allow disabling serialization.

Serialization is not always useful, for example when deriving command line
arguments from a configuration.  This change provides a way to turn it off,
which removes the need to define a bunch of dummy serialization procedures.

Credit goes to Andrew Gierth (RhodiumToad) from #guile for providing the
solution.  Thank you!

* gnu/services/configuration.scm (define-configuration-helper): New procedure.
(define-configuration) <no-serialization>: New syntactic keyword.  Use it in a
new pattern.  Refactor the macro so that it makes use of the above helper
procedure.