[USERINIT] Enhancements for starting ReactOS GUI installer or Explorer shell.

[USERINIT] Enhancements for starting ReactOS GUI installer or Explorer shell.

- Transform TryToStartShell() into a StartProcess() function whose aim
  is just to start processes, since there is no extra initialization needed
  for starting a shell process.

- Modify StartInstaller() to call StartProcess() for starting the ReactOS GUI
  installer found on the installation media, from a path automatically expanded
  depending on the ambient CPU architecture.

svn path=/branches/setup_improvements/; revision=75331