[core] Fix for Get/SetThreadUILanguage() not being available on XP

* This addresses the issue reported in #274:
  Unable to access kernel32 DLL: [0x0000007F] The specified procedure could not be found.
* Problem was introduced in 897becd29015e384754a9fa7728cb4ad14a11cb2 which was added for #189.
v1.4.3 (build 385)
[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