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/option.go

@@ -50,6 +50,10 @@ type Opt struct {
 	Kind    Kind
 	Default string
 	Help    string
+	// Metavar overrides the "=PLACEHOLDER" shown after the flag in --help. Empty
+	// means derive it from Kind (N, SIZE, RATIO, ...). Set it only when the
+	// Kind-derived default would mislead, e.g. seed-time is minutes, not a ratio.
+	Metavar string
 	Tag     Tag
 	Enum    []string // valid values when Kind == Enum
 	// Min is the inclusive lower Int/Size bound. The zero value enforces a
@@ -103,7 +107,7 @@ var options = []Opt{
 
 	// --- bittorrent ---
 	{Long: "torrent-file", Short: 'T', Kind: Str, Help: "path to a .torrent file", Tag: BitTorrent},
-	{Long: "seed-time", Kind: Float, Help: "minutes to seed after completion (0 = no seeding)", Tag: BitTorrent},
+	{Long: "seed-time", Kind: Float, Metavar: "MIN", Help: "minutes to seed after completion (0 = no seeding)", Tag: BitTorrent},
 	{Long: "seed-ratio", Kind: Float, Default: "1.0", Help: "stop seeding at this share ratio (0 = unlimited)", Tag: BitTorrent},
 	{Long: "bt-stop-timeout", Kind: Int, Default: "0", Min: 0, Help: "stop a torrent with no download progress for N seconds (0 = off)", Tag: BitTorrent},
 	{Long: "bt-metadata-timeout", Kind: Int, Default: "60", Min: 0, Help: "stop a magnet that can't fetch metadata in N seconds (0 = off)", Tag: BitTorrent},