progress: reset flags at transfer start

When an easy handle is used to download an URI which has no
Content-Length header (or equivalent) after downloading an URI which
does, the value from the previous transfer is reused and returned by
CURLINFO_CONTENT_LENGTH_DOWNLOAD. This is because the progress flags
(used to determine whether such a header was received) are not reset
between transfers.

Bug: http://curl.haxx.se/bug/view.cgi?id=3370895