BUG/MEDIUM: mux-fcgi: Don't request more room if mux is waiting for more data

BUG/MEDIUM: mux-fcgi: Don't request more room if mux is waiting for more data

A mux must never report it is waiting for room in the channel buffer if this
buffer is empty. Because there is nothing the application layer can do to
unblock the situation. Indeed, when this happens, it means the mux is
waiting for data to progress. It typically happens when all headers are not
received.

In the FCGI mux, if some data remain in the RX buffer but the channel buffer
is empty, it does no longer report it is waiting for room.

This patch should fix the issue #2150. It must be backported as far as 2.6.