add example systemd service file

Making use of systemd socket notification feature added in ff270e6d5.
amend comments in upstart config file + disable "console log"

Hopefully avoid some confusion for RHEL6 users which have an old upstart
version.
collectd(1): mention upstart & systemd support
Merge pull request #814 from mfournier/upstart-systemd-examples

upstart and systemd doc & examples