all: clean up minor issues found by staticcheck

Use bytes.Equal instead of comparing bytes.Compare results to zero,
avoid Yoda conditions, drop a redundant zero slice capacity, use
strings.ReplaceAll, build acme error strings with a strings.Builder,
and lowercase an error string in ocsp. No behavior changes.

Change-Id: I5d2b3fa5c241adc75e61be0f2d2d7af1eb9e8d66
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/803200
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

ssh/handshake_test.go

@@ -195,7 +195,7 @@ func TestHandshakeBasic(t *testing.T) {
 		}
 
 		want := []byte{msgRequestSuccess, byte(i)}
-		if bytes.Compare(p, want) != 0 {
+		if !bytes.Equal(p, want) {
 			t.Errorf("message %d: got %v, want %v", i, p, want)
 		}
 	}