Merge MySQL 5.5.53

https://github.com/percona/percona-server/pull/1148

Merge MySQL 5.5.53

Upstream secure_file_priv changes (https://dev.mysql.com/doc/relnotes/mysql/5.5/en/news-5-5-53.html) and our extension for the same (https://www.percona.com/doc/percona-server/5.5/management/secure_file_priv_extended.html) reconciled as follows:
- empty secure-file-priv becomes an alias for NULL value: both disable LOAD_FILE(), LOAD DATA INFILE, and SELECT INTO OUTFILE
- it is no longer possible to disable security checks by omitting the option as that would take some default value, depending on the package used. Instead, --secure-file-priv='' (or =/) should be used.