speakup: add unicode variant of /dev/softsynth

This adds /dev/softsynthu, along /dev/softsynth, which emits output in
UTF-8 encoding, thus allowing to support 16bit characters.  Most of the
code is shared, only the read function has to behave differently in
latin1 and in unicode mode.  Since Linux only supports 16bit characters,
we can just hardcode the UTF-8 encoding.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Chris Brannon <chris@the-brannons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>