ntp: update 4.2.8p6

QA error fix:
ERROR: QA Issue: ntp: Files/directories were installed but not shipped in any package:
  /usr/libexec

CVES addressed:

Bug 2948 / CVE-2015-8158
Bug 2945 / CVE-2015-8138: origin: Zero Origin Timestamp Bypass
Bug 2942 / CVE-2015-7979: Off-path Denial of Service (DoS) attack on authenticated broadcast mode
Bug 2940 / CVE-2015-7978: Stack exhaustion in recursive traversal of restriction list
Bug 2939 / CVE-2015-7977: reslist NULL pointer dereference
Bug 2938 / CVE-2015-7976: ntpq saveconfig command allows dangerous characters in filenames
Bug 2937 / CVE-2015-7975: nextvar() missing length check
Bug 2936 / CVE-2015-7974: Skeleton Key: Missing key check allows impersonation between authenticated peers
Bug 2935 / CVE-2015-7973: Deja Vu: Replay attack on authenticated broadcast mode
Bug 2947 / CVE-2015-8140: ntpq vulnerable to replay attacks
Bug 2946 / CVE-2015-8139: Origin Leak: ntpq and ntpdc, disclose origin

NTP-4.2.8p5
NtpBug2956: Small-step/Big-step CVE-2015-5300

Bug #282...