[USETUP][SETUPLIB]: Code refactoring: - Move several global setup variables into a structure "USETUP_DATA", similar to the syssetup structure "SETUPDATA" (or the WIP 1st-stage installer structure of the same name), so that these variables can be set easily by different helper setup functions; - Move CheckUnattendedSetup() and GetSourcePaths() to setuplib and make CheckUnattendedSetup() use the USETUP_DATA structure; - Add a LoadSetupInf() function that loads the txtsetup.sif file (factoring out the corresponding code in usetup); - Add a InstallSetupInfFile() function (that I'll certainly rename later on) whose purpose is to create a valid "$winnt$.inf" setup installation file in the ReactOS\system32 directory, which should help the 2nd-stage installer to correctly retrieve the source installation media we used during 1st-stage, and contain the unattended setup lines copied from unattend.inf. This is done in a Windows-compatible way. - Add the lib/infsupp.c and lib/setuplib.c to compilation.
[SETUPLIB][USETUP]: Split the registry helper code into registry utility functions (create/mount/unmount/verify hives) and registry creation code (the rest: initializing the system hives) and move it in the setup library. This commit won't correctly compile yet due to problems related to the interfacing of the INF functions.