Bring in all of the mh-e support that is actually used

to be compiled by default, and garbage collect the stuff that is not used.

Pulled in from nmh. Thanks to Ken Hornstein.

Concerning mmh only:
- The FIXME comments about the missing checks for existing drafts.
- mh-e support of mmh is not much more than providing the -build switches.
  It might get dropped in the future.