BUG/MEDIUM: http-client: Ask for more room when request data cannot be xferred

BUG/MEDIUM: http-client: Ask for more room when request data cannot be xferred

When the request payload cannot be xferred to the channel because its buffer
is full, we must request for more room by calling sc_need_room(). It is
important to be sure the httpclient applet will not be woken up in loop to
push more data while it is not possible.

It is only an issue with large POSTs, when the payload is streamed.

This patch must be backported as far as 2.6. Note that on 2.6,
sc_need_room() only takes one argument.