Hi Christian, on second thought I don't have too much to say about this, other than style comments: take them or leave them.
+/* Only use this function internally. */ This comment isn't all that useful. You might indicate that quotes are only needed by CertNameToStr when the string type is CERT_X500_NAME_STR, but that might be clear enough from the code. Also, since CertRDNValueToStr doesn't need quote_string, you should probably invert the order in which they appear in the file.
Thanks, --Juan