security/vuxml: Document multiple vulnerabilities in irc/irssi

Security: CVE-2018-5205
Security: CVE-2018-5206
Security: CVE-2018-5207
Security: CVE-2018-5208

PR: 224954
Reported by: tj@mrsk.me (email)
Reported by: David O'Rourke <dor.bsd@xm0.uk>