Given that we control all call sites, the only way that this can fail is during
OOM. Also remove the code in csync itself to make sure that it's obvious that
any new error case wouldn't be handled by call sites.
Before we only had c_utf8_to_locale, but now functionality is needed to
convert a path to UNC before converting it. That does c_utf8_path_to_locale
now, while c_utf8_string_to_locale only converts the plain string, ie.
to generate wide char strings for output.
