Windows-path safety check now returns a reason code instead of a boolean

The function that checks whether a file path is unsafe on the current platform now returns a specific reason code instead of a plain true/false. Possible reasons include nt_device_namespace, colon_past_drive_position, tilde_digit, device_path_prefix, trailing_dot_or_whitespace, dos_device_suffix, dot_run_segment, and unc_or_webdav_form.