securityonion-setup: increase MySQL open files limit

[dougburks]

Reported here:
https://groups.google.com/d/topic/security-onion/l-1-C6EPKz0/discussion

Fix should look something like this:

mkdir -p /lib/systemd/system/mysql.service.d
cat << EOF >> /lib/systemd/system/mysql.service.d/limit_nofile.conf
[Service]
LimitNOFILE=90000
EOF
systemctl daemon-reload
systemctl restart mysql

[dougburks]

submitted for testing:
https://groups.google.com/d/topic/security-onion-testing/Bpdvy_j5RSY/discussion

[dougburks]

published:
https://blog.securityonion.net/2018/09/securityonion-setup-20120912.html