emacs/mua: Let user specify which parts get a header in citations.

Add a customizable function specifying which parts get a header when
replying, and give some sensible possiblities. These are,

1) all parts except multipart/*. (Subparts of a multipart part do
receive a header button.)

2) only included text/* parts.

3) Exactly as in the show buffer.

4) None at all. This means the reply contains a mish-mash of all the
original message's parts.

In the test suite we set the choice to option 4 to match the
previous behaviour.