Fix typo in `restrict` setting of ntp configuration

This typo broke the intended restrict configuration, allowing public
access to the NTP server if $ntp_manage was enabled (and e.g. no
firewall was additionally configured).
ntp: restrict to localhost for IPv6, too
Add documentation for ntp config
Harden ntp configuration as per Team Cymru recommendations

http://www.team-cymru.org/ReadingRoom/Templates/secure-ntp-template.html
Fix indentation for ntp code comments
Release version 1.0.1