[USETUP]: Implement offline ReactOS registry initialization in USetup (equivalent of mkhive, but using OS functionality). The rationale is as follows. We now have the 1st-stage ReactOS setup running with its own registry SYSTEM hive, similarly to regular ROS running instances (livecd, regular installation...). The ReactOS-specific SetInstallPathValue() hack, introduced in r3794 and r3795, is removed. This hack told the kernel that, during the setup, it should "switch" the used registry hives and instead use the ones of the ROS installation being prepared. This was really hackish because this means, mixing between registry settings used only for the setup running instance, that could use different registry settings than the ones that should be set for the ROS installation being actually performed. Also, note that in the case of a 1st-stage GUI setup, consisting in running the LiveCD + the GUI setup program, this situation would be untenable. Note also that for people willing to use the Setup*** functions exported by setupapi.dll to parse the registry INF files to initialize the registry of the ROS installation being prepared, this would be impossible either. Hence the need to have offline registry modification functionality.