[syslinux] fix support for recent Syslinux versions

* Detect Syslinux version from isolinux.bin (Closes #272)
* Download required Syslinux v5+ files where needed (Closes #165)
* Also fix an issue where using a Syslinux based ISO didn't switch to ISO mode
* Also alter DownloadFile and ExtractISOFile to return a size
[core] detect, report and warn when multiple partitions are in use

* Closes #191
* Also fix various minor warnings