Support for encoding format=flowed (RFC 2646)

[Simon Josefsson <jas@extundo.com>]

I have checked in preliminary format=flowed support.  The missing
functionality right now is doing the right thing when following up on
an article.

How to use: Enable `use-hard-newlines' (see emacs manual) in the
message buffer, and the code should fill and format=flow encode your
article according to the soft/hard newlines in the buffer.  Hint: I
find `use-hard-newlines' useless without something similar to M-x
longlines-show-hard-newlines RET from longlines.el.

(I also changed the format=flowed decoding code to flow the message to
the width of the window.)

As you can see, the above
paragraphs are filled nicely
while this one sentences isn't.
This is because I hit RET after
each of these lines, without
using M-q on the paragraph.