[misc] set rufus-next to 1.3.4, update copyright year

* Copyright year in trhe about dialog had not been updated
[syslinux] add detection for Syslinux/Isolinux v5.0

* This is not full Syslinux v5.0 support, but only to prevent the (vesa)menu.c32 message from displaying
* Uses the mandatory inclusion of ldlinux.c32 in Isolinux v5.x for the detection
* Part of a fix for #142