Keep asking on an older bus module and name the real remedy for each grant not held (review of hq issue 353)

A bus module older than its in_force answer read as not held, so a merge
before mesh-catalog #166 is live would silence every ask until a person's
bus upgrade. Such an answer now falls back to the check main makes (is the
list sent to the bus's machine the one composed now), says the reload is
not confirmed, and each case names its own remedy: push <node> for a list
behind or not allowing it, a ~5 s wait for a reload, the bus upgrade when
an older module's list cannot be judged.

cmd/mesh-controller/asker.go

@@ -213,6 +213,8 @@ type asker struct {
 
 	saidNoRouter bool
 	saidNoGrant  bool
+	// saidGrantCaveat is whether a grant held with a caveat was said, so it is said once.
+	saidGrantCaveat bool
 
 	mu     sync.Mutex
 	nudged chan struct{}
@@ -317,6 +319,13 @@ func (a *asker) reconcile(ctx context.Context) error {
 			}
 			return a.sayUnasked(ctx, unasked, "the bus does not hold the controller's grant to ask yet: "+why)
 		}
+		if why != "" && !a.saidGrantCaveat {
+			// Held, with a caveat worth one line (an older bus module cannot confirm the reload).
+			a.logf("the controller asks: %s", why)
+			a.saidGrantCaveat = true
+		} else if why == "" {
+			a.saidGrantCaveat = false
+		}
 		a.saidNoGrant = false
 	}
 	channels := ""