emacs: use mark instead of point-max in MML quoting.

As Aaron explains in id:"m2vco72tf3.fsf@wal122.wireless-pennnet.upenn.edu"

  Using point-max would include the signature in the quoting as well.
  It would probably be fairly odd to want to put an MML tag in one’s
  signature, but that doesn’t mean that we should break that usage.

We had to use point-max in the 0.11.1 bug-fix release, because the
mark functionality was added post 0.11.