Fix: refactorize llx_c_currencies and currency symbol

htdocs/core/class/html.form.class.php

htdocs/core/class/translate.class.php

htdocs/core/lib/functions.lib.php

htdocs/core/lib/functions2.lib.php

htdocs/core/lib/pdf.lib.php

htdocs/install/mysql/data/llx_c_currencies.sql

htdocs/install/mysql/migration/3.1.0-3.2.0.sql

htdocs/install/mysql/tables/llx_c_currencies.sql