This makes applications with custom callbacks work properly, and fixes
'make test' in a win9x mode.

Previously the shortcuts for cut, copy, paste, undo, redo, and select
all were being handled during the WM_CHAR message.  These tests show
that these shortcuts should be handled with the WM_KEYDOWN message
instead.

Existing tests show that MMIO_ALLOCBUF flag is dropped if a valid buffer
is being passed to mmioOpen, that means the buffer is valid in that case.