services: network-manager: Set service canonical-name to NetworkManager.

According to the semantics in [1], 'networking should be a "virtual service"
and NetworkManager its canonical-name. This does not influence
existing services and they should continue to use the 'networking symbol.

One visible change is that 'herd status' doesn't show 'networking' anymore,
instead listing 'NetworkManager' in its place but both symbols are can be used
to start and stop the same service.

Note: Though the symbol NetworkManager doesn't really conform with the overall kebab-case
used throughout Guix, this is intentional as we really want to make it clear that
that the symbol NetworkManager really refers to the software called NetworkManager,
since it's a canonical name here. (rather than risk misleading the user to interpret
the symbol network-manager as a symbol for some unspecific network management software)

[1]: https://www.gnu.org/software/shepherd/manual/html_node/Jump-Start.html

* gnu/services/networking.scm (network-manager-shepherd-service): Make 'networking a
virtual service and set 'NetworkManager as its canonical name.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/networking.scm

@@ -1229,7 +1229,7 @@ (define (network-manager-shepherd-service config)
           (vpn  (vpn-plugin-directory vpn-plugins)))
       (list (shepherd-service
              (documentation "Run the NetworkManager.")
-             (provision '(networking))
+             (provision '(NetworkManager networking))
              (requirement `(user-processes dbus-system loopback
                             ,@shepherd-requirement
                             ;; TODO: iwd? is deprecated and should be passed