Document squid -- multiple vulnerabilities

PR: 208939
Reported by: Pavel Timofeev <timp87@gmail.com>
Security: CVE-2016-4054
Security: CVE-2016-4053
Security: CVE-2016-4052
Security: CVE-2016-4051
Security: https://vuxml.FreeBSD.org/freebsd/e05bfc92-0763-11e6-94fa-002590263bf5.html