* tree.c include/libxml/tree.h: modified the existing APIs
  to handle XHTML1 serialization rules automatically, also add
  xmlIsXHTML() to libxml2 API. Some tweaking to make sure
  libxslt serialization uses it when needed without changing
  the library API.
* test/xhtml1 result/noent/xhtml1 result/valid/xhtml1.xhtml
  result/xhtml1: added a new test specifically for xhtml1 output
  and updated the result of one XHTML1 test
Daniel