Fix #79200: Some iconv functions cut Windows-1258

Fix #79200: Some iconv functions cut Windows-1258

To cater to potentially state-dependent encodings, we have to reset the
conversion descriptor into its initial shift state to properly finish
the conversion.  Furthermore, state-dependent encodings may not show
progress when comparing `in_left` before and after the conversion; we
rather have to see whether `out_left` has decreased.  Also we have to
cater to the fact that the final potentially state resetting call does
not signal failure, but we still have to break respective loops
afterwards.