BUG/MEDIUM: mux-h1: Notify the stream waiting for TCP splicing if ibuf is empty

BUG/MEDIUM: mux-h1: Notify the stream waiting for TCP splicing if ibuf is empty

When a stream-interface want to use the TCP splicing to forward its data, it
notifies the mux h1. We will then flush the input buffer and don't read more
data. So the stream-interface will not be notified for read anymore, except if
an error or a read0 is detected. It is a problem everytime the receive I/O
callback is called again. It happens when the pipe is full or when no data are
received on the pipe. It also happens when the input buffer is freshly
flushed. Because the TCP splicing is enabled, nothing is done in h1_recv() and
the stream-interface is never woken up. So, now, in h1_recv(), if the TCP
splicing is used and the input buffer is empty, the stream-interface is notified
for read.

This patch must be backported to 1.9.