Again, updates for files and directories handling.

* [DIR_EXT,EXT] Add directory_files2/2 to the public predicates.
* [DIR_EXT,EXT] Extended directory_files2/2 to return absolute file name i.o. relative ones.
* [DIR_EXT,SC] Simplified directory_files/3 using the new directory_files2/2.
* [FILE_EXT,EXT] Rewrote merge_into_one_file/2 based on feedback from JW on the swipl mailing list.
* [RDF_SERIAL,FIX] rdf_save2/2 did not work for the XML/RDF format.
* [XML_STREAM,EXT] Using the tmp file again (fixed by JW).