Fix systemd behavior when the config file is missing.

This was causing problems, for example, when only installing collectd-core which does not provide a config file. The old init script handles this gracefully and so does the service file now.

Thanks to @bzed for reporting this issue !

Preparing for 5.5.0-3

It's Intel-hardware specific.

Upstream version 5.5.0

* Build-depend on debhelper (>= 7.0.50~) and drop dpatch.
* Renamed patch files from *.dpatch to *.patch and drop the dpatch header.
* Changed debian/patches/00list to debian/patches/series.
* Drop all patch operations from debian/rules.

Thanks to YunQiang Su for an initial patch.
Closes: #688719

Thanks to Frans Spiesschaert.
Closes: #763783

Thanks to Otto Kekäläinen for reporting this.
Closes: 781980

Now that part of the headers have been moved to the daemon/
subdirectory, this bit needs to be slightly more sophisticated.

Mostly based on recommendations from:
https://wiki.debian.org/Teams/pkg-systemd/Packaging