[SETUPLIB][USETUP] Transform the existing ConcatPaths() function into a variadic...

[SETUPLIB][USETUP] Transform the existing ConcatPaths() function into a variadic function, and derive a CombinePaths() from it (plus their equivalent taking va_list).

This allows building concatenated paths with an arbitrary number of separated components.
- Use the newly-introduced CombinePaths() and ConcatPaths() functions.
- Fix also few comments, and place some UNICODE_NULLs here & there.

svn path=/branches/setup_improvements/; revision=74637
svn path=/branches/setup_improvements/; revision=74640