speakup: extend synth buffer to 16bit unicode characters

This extends the synth buffer slots to 16bit, so as to hold 16bit
unicode characters.

synth_buffer_getc and synth_buffer_peek now return 16bit characters.
Speech synthesizers which do not support characters beyond latin1 can
use the synth_buffer_skip_nonlatin1() helper to skip the non-latin1
characters before getting or peeking. All synthesizers are made to use
it for now.

This makes synth_buffer_add take a 16bit character. For simplicity for
now, synth_printf is left to using latin1 formats and strings.
synth_putwc, synth_putwc_s, synth_putws and synth_putws_s helpers are
however added to put 16bit characters and strings.

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>