ui/ncurses: Ensure boot option labels are displayable as menu items

ncurses' new_item() expects the name parameter to be a "printable
string", by converting it to a wchar *, and checking that each character
is printable with iswprint(). If it fails, we won't see a boot option at
all.

This change introduces a function to convert the label into something we
know is printable, and valid UTF-8. If mbstowcs fails, we replace it
with a generic 'Invalid option' label. If we encounter a valid multibyte
string with unprintable characters, we replace those with U+fffd
REPLACEMENT CHARACTER.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>