identification: Add hash() method

Compared to hashing the encoding we can ignore string types of RDNs when
hashing DNs, making hash() compatible to equals() that does the same.

Fixes #991.

src/libstrongswan/utils/identification.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2009 Tobias Brunner
+ * Copyright (C) 2009-2015 Tobias Brunner
  * Copyright (C) 2005-2009 Martin Willi
  * Copyright (C) 2005 Jan Hutter
  * Hochschule fuer Technik Rapperswil
@@ -218,6 +218,14 @@ struct identification_t {
 	 */
 	id_type_t (*get_type) (identification_t *this);
 
+	/**
+	 * Create a hash value for this identification_t object.
+	 *
+	 * @param inc		optional value for incremental hashing
+	 * @return			hash value
+	 */
+	u_int (*hash) (identification_t *this, u_int inc);
+
 	/**
 	 * Check if two identification_t objects are equal.
 	 *