We need to reset the default button using DM_SETDEFID too, otherwise
calling DM_SETDEFID later, when setting the new default button, seems to
restore BS_DEFPUSHBUTTON on the previous default button (but only if its
ID is positive, which probably explains why this bug went unnoticed for
so long), resulting in having 2 buttons with BS_DEFPUSHBUTTON in the
dialog.
Closes#19245.
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