Revert our temporary fix for "Bug#40360 Binlog related errors with binlog off"
(r2935, r2936) and deploy MySQL's one, but put the function
mysql_bin_log_is_engaged() inside mysql_addons.cc instead of in mysql's log.cc
and use a different name for it so there is no collision when MySQL adds this
function in log.cc.

[note from the future: the windows part of this patch went into r2947]

Approved by:	Marko (https://svn.innodb.com/rb/r/41/)