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.

httpdl/segment.go

@@ -55,27 +55,3 @@ func makeSegments(total, minSplit int64, conns int) []seg {
 	}
 	return segs
 }
-
-// maxAutoConns is the ceiling --auto-split scales to. aria2 caps
-// max-connection-per-server (-x) at 16, so a got-chosen count honours the same
-// per-server limit rather than inventing a looser one.
-const maxAutoConns = 16
-
-// autoConns picks a connection count from the file size, used only when
-// --auto-split is set: one connection per min-split-size of content, at least 1
-// and at most maxAutoConns. It is the got-only stand-in for hand-tuning -x/-s,
-// and because the count never exceeds total/minSplit, makeSegments yields exactly
-// that many balanced segments.
-func autoConns(total, minSplit int64) int {
-	if minSplit < 1 {
-		minSplit = 1
-	}
-	n := total / minSplit
-	if n < 1 {
-		n = 1
-	}
-	if n > maxAutoConns {
-		n = maxAutoConns
-	}
-	return int(n)
-}