systemd.bbclass: automatically extend FILES_* for systemd packages

* Add files found in SYSTEMD_SERVICE and add service files referenced by 'Also='
  (and 'Conflicts=' in case of one service for links to /dev/null) recursively.
* For *.socket files the corresponding '*.service' and '*@.service are packed
* In case a file set in SYSTEMD_SERVICE does not exist, build is aborted with an
   error message.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>