[misc] use "in-place" option when running sed

* Also remove unused "Version" entry
[syslinux] improve handling of 4.x C32 files

* Download the files from files/syslinux-4.07 on the server
* Save local copy in rufus_files/syslinux-4.07
* Move menu.c32/vesamenu.c32 check to BootCheck()