Update the autostart file when required

[umbynos]

In #781 we moved the implementation of the auto-start feature of the agent directly in the agent itself. This was done not considering that there could be an old auto-start file already present on the user system. The auto-start file is not updated and still tries to run the old version of the Agent (or if the old version is manually removed, it runs nothing).
A solution would be to check if the auto-start file is up-to-date (instead of just checking if the file is present). If it is not updated, replace it with an updated version.