Rename ssl_conf_datagram_packing() to ssl_set_datagram_packing()

The naming convention is that functions of the form mbedtls_ssl_conf_xxx()
apply to the SSL configuration.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index c86a0f9..e7f7ea4 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1842,8 +1842,8 @@
  *                 are currently always sent in separate datagrams.
  *
  */
-void mbedtls_ssl_conf_datagram_packing( mbedtls_ssl_context *ssl,
-                                        unsigned allow_packing );
+void mbedtls_ssl_set_datagram_packing( mbedtls_ssl_context *ssl,
+                                       unsigned allow_packing );
 
 /**
  * \brief          Set retransmit timeout values for the DTLS handshake.