Merge FreeBSD SA-20:04-05/07-09 and EN-20:04/06

TCP IPv6 SYN cache kernel information disclosure [SA-20:04.tcp]
Insufficient oce(4) ioctl(2) privilege checking [SA-20:05.if_oce_ioctl]
Incorrect user-controlled pointer use in epair [SA-20:07.epair]
Kernel memory disclosure with nested jails [SA-20:08.jail]
Multiple denial of service in ntpd [SA-20:09.ntp]
Missing pfctl(8) tunable [EN-20:04.pfctl]
Incorrect checksum calculations with IPv6 extension headers [EN-20:06.ipv6]