LightDM needs better log file rotation

[Alexandros Frantzis]

At the moment when LightDM starts a process (e.g. unity-system-compositor) it moves any old log file to *.old and starts logging from scratch. The problem is that for long running systems the log files can get quite large and there is no mechanism to restrict their size. LightDM needs a better way to handle log files, possibly by using an external tool like logrotate.

This becomes more important since we plan to increase the volume of log output produced by unity-system-compositor in order to make it easier to diagnose problems seen in the wild.