Sebastien Willemijns reported bug #1603712

(http://curl.haxx.se/bug/view.cgi?id=1603712) which is about connections
getting cut off prematurely when --limit-rate is used. While I found no such
problems in my tests nor in my reading of the code, I found that the
--limit-rate code was severly flawed (since it was moved into the lib, since
7.15.5) when used with the easy interface and it didn't work as documented so
I reworked it somewhat and now it works for my tests.