Add type_handler_xpath_nodeset and remove XPATH_NODESET

In item_xmlfunc.cc there is a number of tests like:

  if (item->type() == Item::XPATH_NODESET)

We'll modify this code in a more Type_handler-ish style, as follows:

• Remove XPATH_NODESET from the enum Item::Type.

• Add a new type handler declaration in item_xmlfunc.cc:

  static Type_handler_long_blob type_handler_xpath_nodeset;

• Overload Item_nodeset_func::type_handler() and Item_nodeset_func::fixed_type_handler() to return &type_handler_xpath_nodeset.

• Change all tests for XPATH_NODESET to a test like this:

  if (item->fixed_type_handler() == &type_handler_xpath_nodeset)

Additionally, we'll do the following cleanups:

• Rename the method Item::val_nodeset() to a more generic Item::val_raw(). It will be later used by other non-traditional data types.
• Remove XPATH_NODESET_CMP, as it's not used.