Fix for BUG#13897 "failure to do SET SQL_MODE=N where N is a number > 31" (the original bug's title isn't the simplest
symptom). sys_var::check_set() was wrong. mysqlbinlog makes use of such SET SQL_MODE=N
(where N is interpreted like if SQL_MODE was a field of type SET), so
this bug affected recovery from binlogs if the server was running with certain SQL_MODE values,
for example the default values on Windows (STRICT_TRANS_TABLES); to work around this bug people
had to edit mysqlbinlog's output.