httpdl/cli: drop --auto-split; classify errors by type, not message text

Two Pike cleanups.

--auto-split was a third connection-count policy (beside -x and -s) that added
an extra knob rather than keeping the model minimal. Removing it also retires the
now-dead autoConns/maxAutoConns and the per-host transport cap that existed only
to scale for it; min(split, M*-x) is the sole policy again.

main's exit-code mapping fell back to substring-matching third-party error text
(strings.Contains "connection refused"/"timeout"/...), which rots when a
dependency rewords a message. The typed checks (errors.Is on the syscall errno,
*net.DNSError, net.Error.Timeout, context.DeadlineExceeded) already cover the
real stdlib errors -- verified end to end: refused -> 6, bad host -> 19. The two
cases that genuinely needed the text match are our own errors, now typed
sentinels: httpdl.ErrTimeout (idle timeout) and the bt metadata timeout wrapping
context.DeadlineExceeded.

bt/bt.go

@@ -392,7 +392,9 @@ func (d *Download) awaitInfo(ctx context.Context, t *torrent.Torrent) error {
 	case <-ctx.Done():
 		return ctx.Err()
 	case <-deadline:
-		return fmt.Errorf("timed out fetching metadata after %s", d.opts.MetaTimeout)
+		// Wrap DeadlineExceeded so the exit-code mapping classifies it as a timeout
+		// (2) via errors.Is, rather than matching on the message text.
+		return fmt.Errorf("timed out fetching metadata after %s: %w", d.opts.MetaTimeout, context.DeadlineExceeded)
 	}
 }