* w32term.h (GUISTR, GUI_ENCODE_FILE, GUI_ENCODE_SYSTEM, GUI_FN)
(GUI_SDATA, guichar_t): Macros to abstract out differences between
NTGUI_UNICODE and !NTGUI_UNICODE builds, some moved out of
w32fns.c.
* w32term.c (construct_drag_n_drop): Use the above macros to make
drag-and-drop work for non-ASCII filenames in cygw32 builds.
* w32fns.c (x_set_name, x_set_title): Use the above macros to
properly display non-ASCII frame titles in cygw32 builds.
* w32fns.c (Fw32_shell_execute): Use the above macros to properly
call ShellExecute in cygw32 builds.
* w32fn.c (Fx_file_dialog): Use the above macros to simplify the
common file dialog code.
* w32fns.c (Ffile_system_info): Remove from cygw32 builds, which
can just use du like other systems.