ntp: security bump to version 4.2.8p7

ntp: security bump to version 4.2.8p7

Fixes:

CVE-2016-1551 - Refclock impersonation vulnerability, AKA:
refclock-peering

CVE-2016-1549 - Sybil vulnerability: ephemeral association attack, AKA:
ntp-sybil - MITIGATION ONLY

CVE-2016-2516 - Duplicate IPs on unconfig directives will cause an
assertion botch

CVE-2016-2517 - Remote configuration trustedkey/requestkey values are not
properly validated

CVE-2016-2518 - Crafted addpeer with hmode > 7 causes array wraparound
with MATCH_ASSOC

CVE-2016-2519 - ctl_getitem() return value not always checked

CVE-2016-1547 - Validate crypto-NAKs, AKA: nak-dos

CVE-2016-1548 - Interleave-pivot - MITIGATION ONLY

CVE-2015-7704 - KoD fix: peer associations were broken by the fix for
NtpBug2901, AKA: Symmetric active/passive mode is broken

CVE-2015-8138 - Zero Origin Timestamp Bypass, AKA: Additional KoD Checks

CVE-2016-1550 - Improve NTP security against buffer comparison timing
attacks, authdecrypt-timing, AKA: authdecrypt-timing

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>