[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 probably 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.

svn path=/branches/setup_improvements/; revision=75518

[USETUP] Close the txtsetup.sif file at the end of the operations.

svn path=/branches/setup_improvements/; revision=75539

base/setup/lib/CMakeLists.txt

@@ -12,6 +12,7 @@ list(APPEND SOURCE
     partlist.c
     registry.c
     regutil.c
+    setuplib.c
     precomp.h)
 
 add_library(setuplib ${SOURCE})