SUPER is not required to change binlog format for session

A user without SUPER privileges can change the value of the
session variable BINLOG_FORMAT, causing problems for a DBA.

This changeset requires a user to have SUPER privileges to
change the value of the session variable BINLOG_FORMAT, and
not only the global variable BINLOG_FORMAT.


mysql-test/suite/binlog/t/binlog_grant.test:
  Adding test to test grants needed for SQL_LOG_BIN and BINLOG_FORMAT.
sql/set_var.cc:
  Adding code to check that user has SUPER permission
  needed to change the value of BINLOG_FORMAT.
sql/set_var.h:
  Adding function sys_var_thd_binlog_format::check()