Fixed bug in JOIN


Docs/manual.texi:
  Changlog
mysql-test/r/join.result:
  New test for join bug
mysql-test/r/null_key.result:
  Fix result after fixing join bug
mysql-test/t/join.test:
  New test for join bug
mysys/my_open.c:
  Fixed bug in counting open files when using many files.
sql/sql_select.cc:
  Fixed join bug