Get rid of some platform-specific functions examining window

Get rid of some platform-specific functions examining window
system and its capabilities.  This is a partial rework of the
2013-04-05 change.
* lisp.h (have_menus_p): Remove prototype.  This function is
replaced with platform-independent window_system_available.
(check_window_system): Move to...
* frame.h (decode_window_system_frame, window_system_available):
...here, add new prototypes.
* frame.c (window_system_available, decode_window_system_frame):
New functions.
(check_window_system): Platform-independent now.
* xterm.h (x_in_use): Remove declaration.
(check_x_frame):
* w32term.h (check_x_frame):
* nsterm.h (check_x_frame): Remove prototypes.  This function
is replaced with platform-independent decode_window_system_frame.
* msdos.c (have_menus_p): Remove.
* nsfns.m (check_window_system, have_menus_p, check_ns_frame):
Remove platform-specific functions.  Use check_window_system,
decode_window_system_frame and check_ns_display_info where
appropriate.  Minor style and comment tweaks.
* w32fns.c (w32_in_use, check_window_system, have_menus_p)
(check_x_frame): Likewise.
* xfns.c (x_in_use, check_window_system, have_menus_p, check_x_frame):
Likewise.
* fileio.c, fns.c, font.c, fontset.c, image.c, menu.c, nsmenu.m:
* nsselect.m, nsterm.m, w32font.c, w32menu.c, xfaces.c, xgselect.c:
* xmenu.c, xselect.c: All related users changed.