Consistently use platform-specific function to detect window system.

Consistently use platform-specific function to detect window system.
* lisp.h (check_window_system): New prototype.  This function is
going to replace check_x, check_w32 and check_ns.
(have_menus_p): Mention msdos.c in comment.
* fontset.c (check_window_system_func): Remove.  Adjust all users.
* fontset.h (check_window_system_func): Remove prototype.
* nsterm.h (check_ns):
* xterm.h (check_x):
* w32term.h (check_w32): Likewise.
* menu.c (Fx_popup_menu): Use check_window_system.
* msdos.c (check_window_system): Define for MS-DOS.
* nsfns.m (check_window_system): Define for NS.  Adjust all users.
* w32fns.c (check_window_system): Likewise for MS-Windows.
* xfns.c (check_window_system): Likewise for X.
* font.c, frame.c, nsmenu.m, nsselect.m, nsterm.m, w32menu.c:
* xfaces.c, xmenu.c: Use check_window_system where appropriate.