Refuse a plans argument that is not a plan's id (issue 561)

Every argument of plans that names a plan (id, wait, stop, close, retry,
go) is checked once where the call becomes a command line, against the
ids the mesh gives plans (plan-<digits>, release-<digits>), so a quote
from a caller never reaches a command or the store (review 75 on #266).

cmd/mesh-controller/seatverbs.go

@@ -336,6 +336,9 @@ func (a *verbArguments) commandLine() ([]string, error) {
 		}
 		return argv, nil
 	case "plans":
+		if err := plainPlanIDs(a.given); err != nil {
+			return nil, err
+		}
 		if r := str("repository"); r != "" {
 			argv := []string{"plans", "--what-if", r}
 			if p := str("paths"); p != "" {