curl: make --progress-bar update the line less frequently

Also, use memset() instead of a lame loop.

The previous logic that tried to avoid too many updates were very
ineffective for really fast transfers, as then it could easily end up
doing hundreds of updates per second that would make a significant
impact in transfer performance!

Bug: http://curl.haxx.se/mail/archive-2013-07/0031.html
Reported-by: Marc Doughty