[Nmh-workers] Reorganizing MIME functions

[Ken Hornstein]

I don't know if anyone noticed, but I added support to burst to allow it
to burst MIME digests (multipart/digest messages, or really anything
containing a message/rfc822 content).  When doing this I noticed some
things:

- The MIME parser & other routines are used by a lot of programs, but they're
  not in libmh.a.  I guess that's historical, but I am thinking that this
  should be cleaned up.  There's some other stuff in there that would need
  to be cleaned up as well, though.

- We don't decend into message/rfc822 types to make any MIME content of
  those messages available; should we?

--Ken