[SETUPLIB][USETUP] Refactor the DoesFileExist() function so that it now looks closer...

[SETUPLIB][USETUP] Refactor the DoesFileExist() function so that it now looks closer to DoesPathExist() and use it almost everywhere.

- Adjust also its callers, adjust OpenAndMapFile() parameters.
- Related to that, simplify IsValidNTOSInstallation() parameters & introduce a IsValidNTOSInstallation_UStr()
  that does the same, but takes a UNICODE_STRING instead.
- Simplify CheckForValidPEAndVendor().

Now only exactly 5 calls use the "old" 'DoesFileExist' syntax, using a temporarily auxiliary function "DoesFileExist_2"...

svn path=/branches/setup_improvements/; revision=74641