Problem: XPath variables didn't work.
Fix: adding variables support,
both user-defined and sp local variables are now supported by XPath.


mysql-test/r/xml.result:
  Adding test case
mysql-test/t/xml.test:
  Adding test case
sql/item_xmlfunc.cc:
  Adding variables support:
  - SP variables with standard XPath syntax:  $i
  - User variables with non-standard syntax:  $@i