Support both systemd and upstart.

Support both systemd and upstart.

This checks appropriate environment variables. When supervised
by either upstart or systemd, collectd will not daemonize but
will signal readyness with the appropriate method.

This allows collectd to be either configured with `expect stop`
in upstart or `Type=notify` with systemd.

The rationale for this is detailed here: http://spootnik.org/entries/2014/11/09_pid-tracking-in-modern-init-systems.html