curl: show headers in bold

The feature is only enabled if the output is believed to be a tty.

-J: There's some minor differences and improvements in -J handling, as
now J should work with -i and it actually creates a file first using the
initial name and then *renames* that to the one found in
Content-Disposition (if any).

-i: only shows headers for HTTP transfers now (as documented).
Previously it would also show for pieces of the transfer that were HTTP
(for example when doing FTP over a HTTP proxy).

-i: now shows trailers as well. Previously they were not shown at all.

--libcurl: the CURLOPT_HEADER is no longer set, as the header output is
now done in the header callback.