Improve the register-hotkey functionality on MS-Windows

Improve the register-hotkey functionality on MS-Windows

* src/w32fns.c (_WIN32_WINNT): Define to 0x0600, needed for
keyboard hook functionality.
Include w32inevt.h, basetyps.h and unknwn.h.
(VK_ANY, WM_WTSSESSION_CHANGE, WTS_SESSION_LOCK): New macros.
(kbdhook): A new struct definition.
(funhook, setup_w32_kbdhook, remove_w32_kbdhook, hook_w32_key)
(check_w32_winkey_state, reset_w32_kbdhook_state): New functions.
(modifier_set): Call check_w32_winkey_state if a Win key was
pressed and the keyboard hook is active.
(w32_wnd_proc): Don't handle Win key combinations if the keyboard
hook is active.  Only register/unregister the hotkeys if the
keyboard hook is not active.  When WM_CREATE is received, call
setup_w32_kbdhook.  When WM_DESTROY is received, call
reset_w32_kbdhook_state.
(lookup_vk_code): When the keyboard hook is active, map
alphanumeric characters to themselves.
(w32_parse_and_hook_hot_key): Renamed from w32_parse_hot_key.  Map
modified keys to VK_ANY if the keyboard hook is active.  Register
Alt-x and Win-x combinations.
(Fw32_shell_execute): Update doc string to reflect new
functionality.  Bypass the code that posts the
WM_EMACS_REGISTER_HOT_KEY message if the keyboard hook is active.
(Fw32_unregister_hot_key): Bypass the code that posts the
WM_EMACS_UNREGISTER_HOT_KEY message if the keyboard hook is active.
(syms_of_w32fns) <w32-pass-lwindow-to-system>
<w32-pass-rwindow-to-system, w32-phantom-key-code>
<w32-lwindow-modifier, w32-rwindow-modifier>: Update doc strings
to reflect the new functionality.
* src/w32console.c (initialize_w32_display): Install the low-level
keyboard hook.
* src/w32inevt.c (key_event): Handle Win-x combinations only if
the keyboard hook is not active.  If the hook is active, use
check_w32_winkey_state instead.
* src/w32term.h (setup_w32_kbdhook, remove_w32_kbdhook)
(check_w32_winkey_state): Add prototypes.
(w32_kbdhook_active): New macro.

* doc/emacs/msdos.texi (Windows Keyboard): Update to reflect the
new functionality.