When binlog_format is STATEMENT and the statement is unsafe before,
the unsafe warning/error message was issued without checking
whether the SQL_LOG_BIN was turned on or not.
      
Fixed with adding a sql_log_bin_toplevel flag in THD to check
whether SQL_LOG_BIN is ON in current session whatever the current is in sp or not.


mysql-test/suite/binlog/r/binlog_unsafe.result:
  Test case result for unsafe warning/error message
mysql-test/suite/binlog/t/binlog_unsafe.test:
  Test case for unsafe message warning/error
sql/set_var.cc:
  Adding a function set_option_log_bin_bit() which specailly handles to
  the change of SQL_LOG_BIN bit in order to set sql_log_bin_toplevel
  according to SQL_LOG_BIN current value at the same time.
sql/sql_class.cc:
  Initialize the flag sql_log_bin_toplevel in THD::init(),
  and add the condition to check whether unsafe ror message was issued.
sql/sql_class.h:
  Add a sql_log_bin_toplevel flag in THD to indicate whether the toplevel SQL_LOG_BIN is