services: nfs: Define rpcbind-shepherd-service at the top level.

services: nfs: Define rpcbind-shepherd-service at the top level.

Attempting to use the 'nfs-service-type' as part of a computed
operating-system definition, the following exception would be thrown:

  ice-9/boot-9.scm:1685:16: In procedure raise-exception:
  ERROR:
    1. &ambiguous-target-service-error:
        service: #<<service> type: #<service-type rpcbind 7f7529853780> value: #<<rpcbind-configuration> rpcbind: #<package rpcbind@1.2.6 gnu/packages/onc-rpc.scm:87 7f75389e78f0> warm-start?: #t>>
        target-type: #<service-type shepherd-root 7f7529396080>
    2. &message: "more than one target service of type 'shepherd-root'"

The problem was that the rpcbind shepherd-service object was dynamically
instantiated every time the rpcbind-service-type would be called, causing
multiple objects in some situations, resulting in the above condition.

* gnu/services/nfs.scm (rpcbind-service-type): Refactor and adjust in a way to
extract...
(rpcbind-shepherd-service): ... this new procedure.