systemd-timesyncd source configuration

For more information about the time servers, refer to their websites or ../../chrony/sources.d which comes with more detailed comments. Chrony is my preferred NTP client/server, but SteamOS happens to come with systemd-timesyncd enabled and using the pool.ntp.org, so I had to revive this at least partially.

It looks like multiple files can be used simultaneously judging by timedatectl show-timesync