services: networking: Add a dependency override mechanism to <static-networking>.

* gnu/services/networking.scm (<static-networking>)[requirement]: New field.
(static-networking-shepherd-service): Don't override requirement for loopback.
(static-networking-service): Expose 'requirement' parameter.  Default to UDEV.
* gnu/services/base.scm (%base-services): Add (requirement '()) for loopback service.
* doc/guix.texi (Networking Services): Document it.