ntp security, bug fix, and enhancement update

[4.2.6p5-22]

• check origin timestamp before accepting KoD RATE packet (CVE-2015-7704)
• allow only one step larger than panic threshold with -g (CVE-2015-5300)
  [4.2.6p5-20]
• validate lengths of values in extension fields (CVE-2014-9297)
• drop packets with spoofed source address ::1 (CVE-2014-9298)
• reject packets without MAC when authentication is enabled (CVE-2015-1798)
• protect symmetric associations with symmetric key against DoS attack (CVE-2015-1799)
• fix generation of MD5 keys with ntp-keygen on big-endian systems (CVE-2015-3405)
• add option to set Differentiated Services Code Point (DSCP) (#1202828)
• add nanosecond support to SHM refclock (#1117702)
• allow creating all SHM segments with owner-only access (#1122012)
• allow different thresholds for forward and backward step (#1193154)
• allow symmetric keys up to 32 bytes again (#1191111)
• don’t step clock for leap second with -x option (#1191122)
• don’t drop packets with source port below 123 (#1171640)
• retry joining multicast groups (#1207014)
• increase memlock limit again (#1053569)
• warn when monitor can’t be disabled due to limited restrict (#1191108)
• use larger RSA exponent in ntp-keygen (#1191116)
• fix crash in ntpq mreadvar command (#1180721)
• move sntp kod database to allow SELinux labeling (#1082934)
• fix typos in ntpd man page (#1195211)
• improve documentation of restrict command (#1213953)