security/vuxml: Document multiple vulnerabilities in www/h2o*

http://blog.kazuhooku.com/2019/08/h2o-version-226-230-beta2-released.html

PR: 239843
Reported by: Kazuho Oku
Approved by: jrm (mentor, implicit)
Security: CVE-2019-9512
Security: CVE-2019-9514
Security: CVE-2019-9515
Sponsored by: SkunkWerks, GmbH