Answer mesh-cli: the control-node's operator is the terminal, everyone else is not (hq ADR 0272)

The records send the operator to "the controller's terminal", and nothing
reached it (hq issue 343). The serving controller now answers each node's
mesh.control.<node>.cli, where only that node's engine may publish, with the
account the engine read from the kernel. A line from the control-node's
operator account runs as the terminal: a fresh process of this binary with
no MESH_VERB, whatever the serving process carries. The same account on any
other node, where agents may run as it, is an ordinary call: the generic
command verb's refusals (one function now, so the two routes cannot drift)
and MESH_VERB=mesh-cli, so a terminal-only change is refused with its
reason. Any other account, root included, runs nothing. Servers are never
run, and an answer over one bus message is cut and says so.

The terminal-only refusals and the usage now name mesh-cli on the
control-node as the way to the terminal.

cmd/mesh-controller/sayable_test.go

@@ -26,6 +26,12 @@ import (
 // shape fails the suite, naming its source. The keeper would say such a summary in words at run time;
 // this is where the producer is made to say it rightly in the first place.
 func TestMain(m *testing.M) {
+	// The process a mesh-cli test runs as a command line: it says the verb and the caller it was given, and
+	// ends (meshcli_test.go).
+	if os.Getenv(echoEnvironment) != "" {
+		fmt.Printf("verb=%q caller=%q\n", os.Getenv("MESH_VERB"), os.Getenv("MESH_CALLER"))
+		os.Exit(0)
+	}
 	conditions.Unsayable = func(o conditions.Observation, field string, r outward.Refusal) {
 		unsaid.note(o, field, r)
 	}