For some reason EM_POSFROMCHAR was returning 0 when the position was
equal to the end of the text, or beyond the end of the text. Instead
it should use the position at the end of the text for both these
cases.  The x value was also seen to be offset by 1 according to the
tests.

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.