MINOR: stconn/mux-h2: Use a iobuf flag to report EOI to consumer side during FF

MINOR: stconn/mux-h2: Use a iobuf flag to report EOI to consumer side during FF

IOBUF_FL_EOI iobuf flag is now set by the producer to notify the consumer
that the end of input was reached. Thanks to this flag, we can remove the
ugly ack in h2_done_ff() to test the opposite SE flags.

Of course, for now, it works and it is good enough. But we must keep in mind
that EOI is always forwarded from the producer side to the consumer side in
this case. But if this change, a new CO_RFL_ flag will have to be added to
instruct the producer if it can forward EOI or not.