services: static-networking: Add ‘scope’ field to <network-route>.

* gnu/services/base.scm (assert-network-route-scope): New procedure.
(<network-route>)[scope]: New field.
(network-set-up/linux)[route-scope->constant]: New procedure.
Use it to pass #:scope to ‘route-add’.
* doc/guix.texi (Networking Setup): Document it.

Fixes: guix/guix#4175
Reported-by: Christopher Baines <mail@cbaines.net>
Change-Id: I24399eca6e691d63fa3d01be564060a3d693d650
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #5507

doc/guix.texi

@@ -22102,6 +22102,22 @@ determined based on @code{destination} or @code{gateway}.
 
 @item @code{gateway} (default: @code{#f})
 IP address (a string) through which traffic is routed.
+
+@item @code{scope} (default: @code{'universe})
+Scope of the route, as a symbol:
+
+@table @code @c see rtnetlink(7)
+@item universe
+global route;
+@item site
+route within the local autonomous system;
+@item link
+route on this link;
+@item host
+route on the local host;
+@item nowhere
+means the destination does not exist.
+@end table
 @end table
 @end deftp