(BUG#2120 sfter merge)


mysql-test/r/derived.result:
  correct results of derived tble EXPLAIN
  test of "Using Index" with derived tables
mysql-test/t/derived.test:
  test of "Using Index" with derived tables
sql/mysql_priv.h:
  way to force derived table save JOIN after execution
sql/sql_derived.cc:
  way to force derived table save JOIN after execution
sql/sql_lex.h:
  way to force derived table save JOIN after execution
sql/sql_select.cc:
  make JOIN::prepare, JOIN::optimize only once for EXPLAIN of derived table