Fix multiple ntp vulnerabilities.

Reviewed by:	roberto (earlier revision), philip
Security:	CVE-2014-9293, CVE-2014-9294
Security:	CVE-2014-9295, CVE-2014-9296
Security:	FreeBSD-SA-14:31.ntp

Differential Revision: https://reviews.freebsd.org/D1343

(cherry picked from commit 1149a2acc4d8123c8f3c58ebf8d8fb827c3261dc)