* test/relaxng/docbook_0.xml: get rid of the dependancy on a locally
  installed DTD
* uri.c include/libxml/uri.h xmlIO.c nanoftp.c nanohttp.c: try to
  cleanup the Path/URI conversion mess, needed fixing in various
  layers and a new API to the uri module which also fixes #306861
* runtest.c: integrated a regression test specific to check the
  URI conversions done before calling the I/O handlers.
Daniel