The brigade seems to end up in a messed up state if something fails
in shutdown, so we clean it up.

Basically, the algorithm to append a converted string to an existing
`smart_str` works by increasing the `smart_str` buffer, to let `iconv`
convert characters until there is no more space, to set the new length
of the `smart_str` and to repeat until there is no more input.

Formerly, the new length calculation has been wrong, though, since we
would have to take the old `out_len` into account (`buf_growth -
old_out_len - out_len`).  However, since there is no need to take the
old `out_len` into account when increasing the `smart_str` buffer, we
can simplify the fix, avoiding an additional variable.

We must not ignore erroneous characters in mime headers, but rather let
iconv_mime_decode() fail in this case, issuing the usual notice
regarding illegal characters.

We have to cater to the possibility that `=?` is not the start of an
encoded-word, but rather a literal `=?`.  If a line break is found
while we're still looking for the charset, we can safely assume that
it's a literal `=?`, and act accordingly.