Support playing on MS-Windows non-ASCII sound files using Unicode APIs.

Support playing on MS-Windows non-ASCII sound files using Unicode APIs.

 src/sound.c [WINDOWSNT]: Include w32common.h and mbstring.h.
 (SOUND_WARNING) [WINDOWSNT]: Include in do..while and improve the
 error message format.  Use message_with_string to have non-ASCII
 file names properly displayed.
 (do_play_sound) [WINDOWSNT]: Use Unicode APIs to play sound files
 when w32-unicode-filenames is non-nil, but not on Windows 9X,
 where these APIs are not available even in UNICOWS.DLL.  Improve
 the format of error messages and include the file name in them
 where appropriate.
 (Fplay_sound_internal) [WINDOWSNT]: Make the MS-Windows branch
 call play-sound-functions, per documentation.
 src/w32.c (w32_get_long_filename, w32_get_short_filename): Constify
 the input file name arguments.
 src/w32.h (w32_get_long_filename, w32_get_short_filename): Update
 prototypes.