Write rfc2231 parameter continuations for long parameters.

Write rfc2231 parameter continuations for long parameters.

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.