Fix some warnings in w32 specific code.

Fix some warnings in w32 specific code.

 src/w32term.c (w32_initialize): Declare the argument of
 set_user_model as const.
 src/w32menu.c <MessageBoxW_Proc>: Fix argument declarations.
 (w32_menu_show): Constify some arguments passed to MessageBox.
 src/w32uniscribe.c (uniscribe_font_driver): Use LISP_INITIALLY_ZERO
 to initialize Lisp objects.
 src/w32font.c (w32font_driver): Use LISP_INITIALLY_ZERO to
 initialize Lisp objects.
 src/frame.c (x_set_frame_parameters) [HAVE_X_WINDOWS]: Declare and
 use variables used only on X under that condition.
 src/fileio.c (Fcopy_file) [!WINDOWSNT]: Don't declare on Windows
 variables not used there.