Fix multiple vulnerabilities of ntp. [SA-16:09]

Fix Linux compatibility layer issetugid(2) system call
vulnerability. [SA-16:10]

Security: FreeBSD-SA-16:09.ntp
Security: FreeBSD-SA-16:10.linux
Approved by: so

Upgrade NTP to 4.2.8p4.

Security: FreeBSD-SA-15:25.ntp
Security: CVE-2015-7871
Security: CVE-2015-7855
Security: CVE-2015-7854
Security: CVE-2015-7853
Security: CVE-2015-7852
Security: CVE-2015-7851
Security: CVE-2015-7850
Security: CVE-2015-7849
Security: CVE-2015-7848
Security: CVE-2015-7701
Security: CVE-2015-7703
Security: CVE-2015-7704, CVE-2015-7705
Security: CVE-2015-7691, CVE-2015-7692, CVE-2015-7702
Approved by: so