instead of looking of NIDs and then using X509V3_EXT_nconf_nid,
instead just pass strings to X509V3_EXT_nconf, which has all the logic for
processing dealing with generic extensions
also process the oid through ln2nid() to retain compatibility.

[rhe: tweaked commit message and added a test case]

https://github.com/ruby/openssl/commit/9f15741331