Add function returning the ordered list of attributes of a DN

* src/dn.cpp (listAttributes): New.
(serialise): Use listAttributes.
* src/dn.cpp, src/dn.h (class DN): Add member function prettyAttributes.

This member function was added to libkleo's copy of DN with e101d885.
Add it here to be able to get rid of libkleo's copy of DN.