[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

base/setup/lib/filesup.h

@@ -51,6 +51,11 @@ DoesPathExist(
 BOOLEAN
 DoesFileExist(
     IN HANDLE RootDirectory OPTIONAL,
+    IN PCWSTR PathNameToFile);
+
+// FIXME: DEPRECATED! HACKish function that needs to be deprecated!
+BOOLEAN
+DoesFileExist_2(
     IN PCWSTR PathName OPTIONAL,
     IN PCWSTR FileName);
 
@@ -64,8 +69,7 @@ NtPathToDiskPartComponents(
 NTSTATUS
 OpenAndMapFile(
     IN HANDLE RootDirectory OPTIONAL,
-    IN PCWSTR PathName OPTIONAL,
-    IN PCWSTR FileName,             // OPTIONAL
+    IN PCWSTR PathNameToFile,
     OUT PHANDLE FileHandle,         // IN OUT PHANDLE OPTIONAL
     OUT PHANDLE SectionHandle,
     OUT PVOID* BaseAddress,