webcit-ng: fixed the way HTTP uploads are fed into the MIME Parser.

mime_parser() expects HEADERS+CONTENT but our h.request_body only contains CONTENT.
The fix: our transaction struct now has h.request_body_with_synth_headers that
begins with a synthetic Content-Type: header followed by a blank line and then the
request body.  h.request_body is now just a pointer into that buffer, starting at
the position where the actual body begins.

This gives us the ability to supply the body with or without headers, without having
to make two copies of it.