Update 4.2.8p2 --> 4.2.8p3

Special thanks to delphij@ for the prep work.

Security: VuXML: 0d0f3050-1f69-11e5-9ba9-d050996490d0
Security: http://bugs.ntp.org/show_bug.cgi?id=2853
Security: https://www.kb.cert.org/vuls/id/668167
Security: http://support.ntp.org/bin/view/Main/SecurityNotice#June_2015_NTP_Security_Vulnerabi