Unix: add uninstall.sh and update install.sh to modify it during install

[densnow]

Note: The URL on line 91 has been changed to download from 14-create-uninstallation-scripts instead of main for testing purposes. This will need to be changed back before merging.

Another couple of points:

• The uninstall script has been added to FILES so will be downloaded if no arg is given. If arg is given install.sh will copy uninstall.sh to the M269 folder. So whichever way the script is run uninstall.sh will end up in the M269 folder; not sure if this is correct based on what the second mode is supposed to be used for.
• The messaging (echoed to the user) and general comments might need some changes for consistency with the original script and just a better "bedside manner".
• The script does not delete itself at the end. For the process of reinstallation I think it would make sense to do this?

[densnow]

I am not sure why I did not realise this before, but reading in between (or directly) the lines of Michels comments and picking up the subtle hints. We do not need to introduce code into the main branch at this time (so close to the 23j release) that could potentially break; it is just so unnecessary. Nobody has the time to test this thoroughly at present and the gains are minimal at best.

We can still have the 14-create-uninstallation-scripts branch to work on and hopefully we can get some adventurous 23j students to help test and contribute towards this script. Thanks for you patience Michel :smile: