MINOR: mux-h2: set CO_SFL_MSG_MORE when sending multiple buffers

MINOR: mux-h2: set CO_SFL_MSG_MORE when sending multiple buffers

Traces show that sendto() rarely has MSG_MORE on H2 despite sending
multiple buffers. The reason is that the loop iterating over the buffer
ring doesn't have this info and doesn't pass it down.

But now we know how many buffers are left to be sent, so we know whether
or not the current buffer is the last one. As such we can set this flag
for all buffers but the last one.