Composer: Make sure to produce CRLF line endings when building messages

Using CRLF line breaks is compulsory per RFC 2822 [1]. Trojita was *not*
actually broken due to sheer luck. What we sent was always wrapped in the
quoted-printable encoding which made sure that there were no stray LF or CR
characters. It came with a drawback, the actual message body came with plenty of
qp-style "=0A" escapes and it also contained excessive artificial line breaks.

After this change, the composer builds reasonably formatted messages again. The
already-existing CRLF line endings from the format=flowed pass are reused (if
possible) and even the totally undecoded message raw format is readable by mere
mortals again.

The only bit which we're doing differently is applying q-p unconditionally. RFC
3676 suggests that it should not be used for mere protecting of the trailing
spaces unless the message contains non-ASCII characters or when it's supposed to
be digitally signed. I don't particularly like the idea of adding an extra code
path for plain-ASCII messages, and given that cryptographic signatures will come
some day anyway, I prefer to apply q-p even on messages where it isn't
necessary.

[1] http://tools.ietf.org/html/rfc2822#section-2.3