cli: trim basic help, drop invented flag, parallelize by default

Surface area, not the machinery, was the problem: 16 flags crowded the
bare --help. Re-tag so basic shows only the 10 outcome-changing flags;
every other flag still works behind --help=<group>.

- cut --bt-metadata-timeout, an invented flag with no real counterpart. Keep
  its 60s magnet-metadata stall as a fixed internal default (bt.go), not a
  user knob.
- fix the parallelism footgun: a bare `got URL` used min(split,x)=1
  connection. When -x is unset, let --split drive per-host connections
  (capped at 16), so `got URL` gets 5 and `-s16` gets 16. An explicit -x
  is honored verbatim, including -x1. Locked by TestHTTPConnDefault.
- -s is the per-download speed dial now, so it moves to basic; -x (a
  per-server cap) moves to the http group. -j help says "files at once,
  not connections within one file".

main.go

@@ -524,11 +524,21 @@ func httpConfig(opts *cli.Options, single bool, overallDL *rate.Limiter) httpdl.
 	if single {
 		out = opts.Str("out")
 	}
+	// max-connection-per-server caps connections per host. aria2's default is 1,
+	// which on its own would clamp --split to a single connection, so a bare
+	// `got URL` would download single-threaded. Honor the literal default of 1
+	// only when the user actually set -x; otherwise let --split drive per-host
+	// parallelism, capped at the -x ceiling of 16. -x explicitly set keeps the
+	// exact min(split, mirrors*x) behavior.
+	perHost := opts.Int("max-connection-per-server")
+	if !opts.IsSet("max-connection-per-server") {
+		perHost = min(opts.Int("split"), 16)
+	}
 	return httpdl.Config{
 		Dir:              opts.Str("dir"),
 		Out:              out,
 		Split:            opts.Int("split"),
-		MaxConnPerServer: opts.Int("max-connection-per-server"),
+		MaxConnPerServer: perHost,
 		MinSplit:         opts.Size("min-split-size"),
 		Tries:            opts.Int("max-tries"),
 		Timeout:          time.Duration(opts.Int("timeout")) * time.Second,
@@ -583,7 +593,6 @@ func btOptions(opts *cli.Options) bt.Options {
 		SeedTime:       time.Duration(opts.Float("seed-time") * float64(time.Minute)),
 		SeedRatio:      opts.Float("seed-ratio"),
 		StopTimeout:    time.Duration(opts.Int("bt-stop-timeout")) * time.Second,
-		MetaTimeout:    time.Duration(opts.Int("bt-metadata-timeout")) * time.Second,
 		SelectFiles:    parseSelect(opts.Str("select-file")),
 		CheckIntegrity: opts.Bool("check-integrity"),
 		DryRun:         opts.Bool("dry-run"),