mesh-cli: give a terminal line the standard input mesh-cli carried, refuse it to any other, keep it nowhere

A secret given at the controller's terminal through mesh-cli (secret accept … --from -) never reached the
line: every line ran with no standard input. A line that runs as the terminal now reads what mesh-cli
carried (at most 64 KiB); an ordinary call carrying any is refused and nothing runs; the calls record keeps
only that some was given, the journal and the answer nothing of it.

cmd/mesh-controller/sayable_test.go

@@ -28,6 +28,11 @@ import (
 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).
+	// The process a mesh-cli test runs as `secret accept`: it reads its value exactly as `secret accept` does
+	// (valueFor) and says whether it is the one the test gave, never the value (meshcli_stdin_test.go).
+	if want := os.Getenv(secretAcceptWants); want != "" {
+		os.Exit(readAsSecretAccept(want, os.Args[1:]))
+	}
 	if os.Getenv(echoEnvironment) != "" {
 		fmt.Printf("verb=%q caller=%q terminal=%v\n", os.Getenv("MESH_VERB"), os.Getenv("MESH_CALLER"), startedAtTheTerminal())
 		os.Exit(0)