cli: simplify option model per pike review

- appendList: one home for the List newline-join rule (was duplicated
  across accumulate and set)
- fold the redundant Int64 getter into Int; drop the truthy helper for
  the shared boolWord vocabulary
- add Opt.Metavar so help derives the value placeholder from data
  instead of branching on the option name (seed-time)

No behavior change; --help output is byte-identical.

cli/help.go

@@ -89,24 +89,31 @@ func flagLabel(o *Opt) string {
 		b.WriteString("    ")
 	}
 	b.WriteString("--" + o.Long)
-	switch o.Kind {
-	case Bool:
-		// no argument shown
-	case Size:
-		b.WriteString("=SIZE")
-	case Int:
-		b.WriteString("=N")
-	case Float:
-		// seed-time is a count of minutes; only seed-ratio is an actual ratio.
-		if o.Long == "seed-time" {
-			b.WriteString("=MIN")
-		} else {
-			b.WriteString("=RATIO")
-		}
-	case Enum:
-		b.WriteString("=" + strings.Join(o.Enum, "|"))
-	default:
-		b.WriteString("=VAL")
+	if m := metavar(o); m != "" {
+		b.WriteString("=" + m)
 	}
 	return b.String()
 }
+
+// metavar is the placeholder shown after a value-taking flag, e.g. the "N" in
+// "--split=N". An explicit Opt.Metavar wins; otherwise it derives from the Kind.
+// Bool takes no value, so its metavar is empty and no "=ARG" is printed.
+func metavar(o *Opt) string {
+	if o.Metavar != "" {
+		return o.Metavar
+	}
+	switch o.Kind {
+	case Bool:
+		return ""
+	case Size:
+		return "SIZE"
+	case Int:
+		return "N"
+	case Float:
+		return "RATIO"
+	case Enum:
+		return strings.Join(o.Enum, "|")
+	default:
+		return "VAL"
+	}
+}