* autoload.cc (MsgWaitForMultipleObjectsEx): Define.
(MsgWaitForMultipleObjects): Remove.
* select.cc (select_stuff::wait): Use MsgWaitForMultipleObjectsEx with
QS_ALLPOSTMESSAGE and, if possible, MWMO_INPUTAVAILABLE flags. Explain
why. Fix a potential crash due to a NULL pointer in WAIT_FAILED case.
(peek_windows): Use filter pattern on NT4. Explain why.
* wincap.h (wincaps::has_mwmo_inputavailable): New element.
* wincap.cc: Implement above element throughout.