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/keys_test.go

@@ -1014,7 +1014,7 @@ func TestKnownHostsParsing(t *testing.T) {
 		input := test.input
 		if strings.Contains(input, rsaKeyToken) {
 			expectedKey = rsaPub
-			input = strings.Replace(test.input, rsaKeyToken, rsaPubSerialized, -1)
+			input = strings.ReplaceAll(test.input, rsaKeyToken, rsaPubSerialized)
 		}
 
 		marker, hosts, pubKey, comment, rest, err := ParseKnownHosts([]byte(input))