i18n currently assume latin1 encoding, which is not enough for most
languages.

This separates out the utf-8 processing of /dev/synthu, and uses it for
a new synth_writeu, which we make synth_printf now use. This has the
effect of making all the i18 messages processed in utf-8.
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20240327115051.ng7xqnhozyii4ik2@beginSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>