Change verify map writes capture option to verify buffer access.

* This option will now toggle on the behaviour to fill undefined buffer contents
  with a marker value, both if they're created without data (it will be zero
  filled instead) or mapped with discard (it will keep the old contents
  instead).
* There were too many hard to find problems or misconceptions about the buffer
  filling for it to be useful. Now it will be opt-in instead.