- Revert changes that dropped calls to %systemd_{pre,post} in the main package
Until we switch to filetriggers these calls are needed when a new version of
systemd introduced a new config file during an update.
- We also introduce a new build conditional "%filetriggers" to identify easily
which parts of the code will become obsolete when we'll switch to file
triggers (WIP). This is important as this is unlikely to happen on SLE.