Get rid of DIRECTORY_SEPARATOR

This is a first tentative of standardizing on `/` as the directory separator (in Windows too...) and render the usage of DIRECTORY_SEPARATOR unneeded.

It **seems** to work but thorough testing is needed, especially under Windows.

I'm unsure of what I did with unit tests: they should be particularly screened and tested
Makes sense: we should then check/convert on entry/exit.

I did the same on PHPMailer, I'm afraid...