Moderate: ntp security, bug fix, and enhancement update (Oracle Linux 7)

[4.2.6p5-28.0.1]
- Bump release to avoid ULN conflict with Oracle modified errata.

[4.2.6p5-28]
- fix buffer overflow in datum refclock driver (CVE-2017-6462)
- fix crash with invalid unpeer command (CVE-2017-6463)
- fix potential crash with invalid server command (CVE-2017-6464)
- add Spectracom TSYNC driver (#1491797)
- fix initialization of system clock status (#1493452)
- fix typos in ntpd man page (#1420453)
- use SHA1 request key by default (#1442083)
- use network-online target in ntpdate and sntp services (#1466947)

[4.2.6p5-27]
- fix CVE-2016-7429 patch to work correctly on multicast client (#1422944)

[4.2.6p5-26]
- don't limit rate of packets from sources (CVE-2016-7426)
- don't change interface from received packets (CVE-2016-7429)
- fix calculation of root distance again (CVE-2016-7433)
- require authentication for trap commands (CVE-2016-9310)
- fix crash when reporting peer event to trappers (CVE-2016-9311)