CVE-2018-2562: fix DoS / data corruption in partitioned MEMORY tables
(Server: Partition)
CVE-2018-2773: warn when --pid-file is in a world-writable location
(BUG#26585560)
CVE-2018-3174: stop server as the mysql user in mysql.init so an
attacker who controls the pid file cannot trick init into killing an
unrelated root-owned process (BUG#27919254)