MINOR: mux-h1: Inherit send flags from the upper layer

MINOR: mux-h1: Inherit send flags from the upper layer

Send flags (CO_SFL_*) used when xprt->snd_buf() is called, in h1_send(), are now
inherited from the upper layer, when h1_snd_buf() is called. First, the flag
CO_SFL_MSG_MORE is no more set if the output buffer is full, but only if the
stream-interface decides to set it. It has more info to do it than the
mux. Then, the flag CO_SFL_STREAMER is now also handled this way. It was just
ignored till now.