ntp: sync ebuild with upstream

We were missing `Conflicts=systemd-timesyncd.service` which has been
added to the upstream ebuild. A few other minor tweaks but nothing big.
ntp: bump to 4.2.8, includes important security fixes

This release includes some important security updates:
http://www.kb.cert.org/vuls/id/852879

Also, support for the ntpdc client in ntpd has been removed upstream.
The ntpq client must be used instead.
coreos-init: include CoreOS version in motd
Merge pull request #1020 from marineam/fixes

NTP 4.2.8, CoreOS version in MOTD