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_test.go

@@ -47,31 +47,6 @@ func TestMakeSegments(t *testing.T) {
 	}
 }
 
-func TestAutoConns(t *testing.T) {
-	const m = int64(20 << 20) // 20 MiB min-split, aria2's default
-	tests := []struct {
-		name     string
-		total    int64
-		minSplit int64
-		want     int
-	}{
-		{"under one min-split is a single connection", 5 << 20, m, 1},
-		{"exactly one min-split", m, m, 1},
-		{"five min-splits", 100 << 20, m, 5},
-		{"caps at maxAutoConns", 10 << 30, m, maxAutoConns},
-		{"rounds down to whole pieces", m*3 + 1, m, 3},
-		{"tiny min-split still capped", 1 << 30, 1 << 20, maxAutoConns},
-		{"zero total is one connection", 0, m, 1},
-	}
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			if got := autoConns(tc.total, tc.minSplit); got != tc.want {
-				t.Errorf("autoConns(%d, %d) = %d, want %d", tc.total, tc.minSplit, got, tc.want)
-			}
-		})
-	}
-}
-
 func TestSegProgress(t *testing.T) {
 	s := seg{start: 100, end: 199} // length 100
 	if s.length() != 100 {