httpdl: add segment work-stealing and --auto-split

an idle worker now steals the back half of the busiest in-flight segment
instead of exiting, so one slow segment no longer drains alone over a single
connection. control file persists stolen tails; resume re-tiles.

add --auto-split (opt-in): derive the connection count from file size (one
per min-split-size, up to 16), overriding -x/-s. default stays at 1
connection for a single server.

main.go

@@ -493,6 +493,7 @@ func httpConfig(opts *cli.Options, single bool, overallDL *rate.Limiter) httpdl.
 		Split:            opts.Int("split"),
 		MaxConnPerServer: opts.Int("max-connection-per-server"),
 		MinSplit:         opts.Size("min-split-size"),
+		AutoSplit:        opts.Bool("auto-split"),
 		Tries:            opts.Int("max-tries"),
 		Timeout:          time.Duration(opts.Int("timeout")) * time.Second,
 		FileAlloc:        opts.Str("file-allocation"),