The paths `C:` (on WIN32) and `/` on other systems was detecting as relative. This meant `BLI_path_abs_from_cwd` would make both paths CWD relative. Also remove duplicate call to BLI_path_is_unc.
Move to a C++ header to allow C++ features to be used there, use the "utils" suffix as it's preferred for new files. Ref !128147