HTTP: Patch CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME

HTTP: Patch CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME

Likely due to some misunderstanding, Fort was managing both of these
variables using one command-line argument (--http.idle-timeout). This
unnecessarily limited the configurability of minimum transfer speeds for
HTTP connections.

--http.idle-timeout is now deprecated, and has been replaced by
--http.low-speed-limit and --http.low-speed-time, which correlate
verbatim to the corresponding curl arguments (CURLOPT_LOW_SPEED_LIMIT
and CURLOPT_LOW_SPEED_TIME).

Thanks to Koen van Hove for reporting this.