Disable the NO_MIXED_MODE option by default but leave it as a commented
entry to make it easy to find. It causes some popular motherboards to
fail and the magic required to make them work isn't widely known.

Requested by: des and others whose email address I've lost
Discussed with: peter, obrien, re@
Tested by: various folks on amd64@, des, kensmith
MTR5_4: Soon