FTP: improve TLS certificate authentication

For each user you can now configure:

- TLS certificate auth
- TLS certificate auth and password
- Password auth

For TLS auth, the certificate common name must match the name provided
using the "USER" FTP command

utils/utils.go

@@ -451,3 +451,16 @@ func GetTLSCiphersFromNames(cipherNames []string) []uint16 {
 
 	return ciphers
 }
+
+// EncodeTLSCertToPem returns the specified certificate PEM encoded.
+// This can be verified using openssl x509 -in cert.crt  -text -noout
+func EncodeTLSCertToPem(tlsCert *x509.Certificate) (string, error) {
+	if len(tlsCert.Raw) == 0 {
+		return "", errors.New("Invalid x509 certificate, no der contents")
+	}
+	publicKeyBlock := pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: tlsCert.Raw,
+	}
+	return string(pem.EncodeToMemory(&publicKeyBlock)), nil
+}