multi_runsingle: CURLOPT_LOW_SPEED_* fix for rate limitation

During the periods of rate limitation, the speedcheck function wasn't
called and thus the values weren't updated accordingly and it would then
easily trigger wrongly once data got transferred again.

Also, the progress callback's return code was not acknowledged in this
state so it could make an "abort" return code to get ignored and not
have the documented effect of aborting an ongoing transfer.

Bug: http://curl.haxx.se/mail/lib-2012-09/0081.html
Reported by: Jie He