Previously, Mutt would truncate long attachment filenames, to avoid
writing an illegal length header line.  This commit is a followup to
4dcb3ba1, where I reverted an incorrect fix for the problem.

rfc2231_encode_string() now returns a list of continuations, with
encoding and continuation number suffixes already appended to the
attribute.  The function tries to keep the line length less than 78
characters, but the code is a bit imprecise as a trade off for
simplicity and readability.

Modify mutt_write_mime_header() to loop through the continuations.