Name the account agents run as on a node, and say whether it can become root

On the control node every agent ran as the operator's account, which has
passwordless sudo, so an agent could become root without a person (hq ADR
0266). A node now names an agent account at the controller's terminal only;
the agent's module declares it never to become root, the node-engine judges
that, and the self-check (DA) raises agent-can-become-root while it does not
hold, so ADR 0259's router can rest on it.

cmd/mesh-controller/facts.go

@@ -269,7 +269,8 @@ func gatherFacts(ctx context.Context, open *stores, busVersion string) (snapshot
 	engines := map[string]bool{}
 	for _, n := range nodes {
 		m := snapshot.Machine{Name: scrub.Machine(n.Name), Length: len(n.Name), Adopted: n.Adopted,
-			AccountHome: scrub.Text(n.AccountHome), NodeEngine: n.HostVersion, PublicDomain: domains[n.Name]}
+			AccountHome: scrub.Text(n.AccountHome), NodeEngine: n.HostVersion, PublicDomain: domains[n.Name],
+			AgentAccount: scrub.Account(n.AgentAccount), AgentAccountHome: scrub.Text(n.AgentAccountHome)}
 		switch n.Account {
 		case "", "root":
 			m.Account = n.Account