Work on generic usage of get_exdir.

htdocs/core/class/html.formfile.class.php

htdocs/core/lib/files.lib.php

htdocs/core/lib/functions.lib.php

.../core/modules/askpricesupplier/doc/pdf_aurore.modules.php

htdocs/core/modules/dons/html_cerfafr.modules.php

htdocs/don/card.php

htdocs/don/class/don.class.php

htdocs/fourn/facture/list.php

htdocs/install/mysql/migration/3.7.0-3.8.0.sql