Serve the uplink seat's verbs from both of its holders (hq ADR 0241)

What a machine resolves through had no tool: the resolver file and who
wrote it, and every link with its default route and the resolvers its
manager knows. Each holder serves the same two node-uplink verbs; the
reading is one text carried by both, held to it by a test, and only
asking the manager for a link's names is each holder's own.

modules/systemd-networkd/cmd/uplink-tools/manager_test.go

@@ -0,0 +1,29 @@
+package main
+
+import "testing"
+
+// systemd-networkd's word on each link's names, read from `networkctl status --all`.
+func TestNetworkctlIsReadPerLink(t *testing.T) {
+	out := `● 1: lo
+                   Link File: n/a
+                Network File: n/a
+                       State: carrier (unmanaged)
+
+● 2: eth0
+                   Link File: /usr/lib/systemd/network/99-default.link
+                Network File: /etc/systemd/network/20-wired.network
+                       State: routable (configured)
+                     Address: 198.51.100.7
+                     Gateway: 198.51.100.1
+                         DNS: 198.51.100.53
+                              198.51.100.54
+              Search Domains: example.net lab.example
+           Activation Policy: up
+`
+	links := parseNetworkctl(out)
+	eth := links["eth0"]
+	if len(links) != 1 || len(eth.Servers) != 2 || eth.Servers[1] != "198.51.100.54" || len(eth.Domains) != 2 ||
+		eth.State != "routable (configured)" || eth.Manager != "systemd-networkd" {
+		t.Fatalf("read %+v", links)
+	}
+}