Document libwww vulnerabilities

PR: 214532
Submitted by: Danilo G. Baio <dbaio@bsd.com.br>
Security: CVE-2009-3720
Security: CVE-2009-3560
Security: CVE-2005-3183
Security: https://vuxml.FreeBSD.org/freebsd/18449f92-ab39-11e6-8011-005056925db4.html