Add md/shaXXX_clone() API

Will be used in the SSL/TLS modules

include/mbedtls/md4.h

@@ -66,6 +66,15 @@ void mbedtls_md4_init( mbedtls_md4_context *ctx );
  */
 void mbedtls_md4_free( mbedtls_md4_context *ctx );
 
+/**
+ * \brief          Clone (the state of) an MD4 context
+ *
+ * \param dst      The destination context
+ * \param src      The context to be cloned
+ */
+void mbedtls_md4_clone( mbedtls_md4_context *dst,
+                        const mbedtls_md4_context *src );
+
 /**
  * \brief          MD4 context setup
  *