Fixed various bugs: setting local variables to NULL, SELECT INTO var now actually
might work, SELECT INTO with not row now gives a "no data" warning (instead of
the "empty query" error), etc.
Updated test cases accordingly.


mysql-test/r/sp-error.result:
  Corrected security syntax for alter/create procedure.
mysql-test/r/sp.result:
  Corrected security syntax for alter/create procedure.
  New tests for setting local variables to null,
  bug fixes for SELECT INTO var, FLUSH TABLES calls (BUG#1862),
  and corrected "no data" warning for SELECT INTO with no rows.
mysql-test/t/sp-error.test:
  Corrected security syntax for alter/create procedure.
mysql-test/t/sp.test:
  Corrected security syntax for alter/create procedure.
  New tests for setting local variables to null,
  bug fixes for SELECT INTO var, FLUSH TABLES calls (BUG#1862),
  and corrected "no data" warning for SELECT INTO with no rows.
sql/lex.h:
  Added SQL_SYM (and added a few _SYM suffixes for new symbols).
sql/sp_head.cc:
  Fixed bug in the item_list copying for "with_wild" cases (list nodes ended
  up in the wrong memroot).
  Catch errors and warnings even if return values is 0 from sub-statements.
  Restore table_list which is zapped by SQLCOM_CREATE_TABLE and INSERT_SELECT.
  Set old table pointers to NULL after sub-statement call (since all tables are
  closed).
sql/sql_class.cc:
  Corrected error message when no rows return by a SELECT INTO var; should
  be a "no data" warning.
sql/sql_lex.h:
  Have to store the original table_list first pointer for some
  sub-statements in SPs.
sql/sql_yacc.yy:
  Corrected SECURITY INVOKER/DEFINER syntax ("SQL" missing), added some _SYM
  suffixes, and fixed valgrind complaints for SP COMMENTs.
  (Also removed some now irrelevant comments.)