multi_socket: set timeout for 100-continue

When libcurl internally decided to wait for a 100-continue header, there
was no call to the timeout function so there was no timeout callback
called when the multi_socket API was used and thus applications became
either completely wrong or at least ineffecient depending on how they
handled the situation. We now set a timeout to get triggered.

Reported by: Ben Darnell
Bug: http://curl.haxx.se/bug/view.cgi?id=3039744