ui/curses: do not assume wchar_t contains unicode

E.g. BSD and Solaris even use locale-specific encoding there.

We thus have to go through the native multibyte representation and use
mbrtowc/wcrtomb to make a proper conversion.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Kamil Rytarowski <n54@gmx.com>
Message-Id: <20190427183307.12796-2-samuel.thibault@ens-lyon.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>