services: Make 'static-networking' extensible.

This allows users to statically define several interfaces.

* gnu/services/networking.scm (<static-networking>)[provision]
[name-servers]: Add default values.
(static-networking-shepherd-service)
(static-networking-etc-files)
(static-networking-shepherd-services): New procedures.
(static-networking-service-type): Change to extend both
SHEPHERD-ROOT-SERVICE-TYPE and ETC-SERVICE-TYPE.
(static-networking-service): Remove default value of #:provision.
Implement using 'simple-service'.
* gnu/services/base.scm (%base-services): Replace
'static-networking-service' call with 'service' form.
* doc/guix.texi (Networking Services): Update documentation.