systemctl: show original contents when editing unit

A comment indicates the start of the new contents of the override file,
and another indicates that lines following it will be discarded once
editing is finished.
The contents of the unit file and drop-ins are listed out after this
last marker.

Adds WRITE_STRING_FILE_TRUNCATE to set O_TRUNC when opening a file.

Thanks to cgzones for providing the required SELinux function calls.

Co-authored-by: Christian Göttsche <cgzones@googlemail.com>