Unify the unspecified argument thing.
diff --git a/OpenSSL/_util.py b/OpenSSL/_util.py
index 1eec17e..a4fe63a 100644
--- a/OpenSSL/_util.py
+++ b/OpenSSL/_util.py
@@ -93,3 +93,8 @@
 else:
     def byte_string(s):
         return s
+
+
+# A marker object to observe whether some optional arguments are passed any
+# value or not.
+UNSPECIFIED = object()