BUG/MEDIUM: mux-h1: Fix again 0-copy forwarding of chunks with an unknown size

BUG/MEDIUM: mux-h1: Fix again 0-copy forwarding of chunks with an unknown size

There is still an issue with zero-copy forwarding of chunks with an unknown
size. It is possible for a producer to fill the sapce reserved for the CRLF
at the end of the chunk. The root cause is that this space is not accounted
in the iobuf offset. So, from the producer point of view, the space may be
used. We can also argue the current design for iobuf is not well suited for
this case. Instead of using a pointer on the consumer's buffer, it could be
easier to use a custom buffer built on top of the consumer one, via a call
to b_make(), with the size, head and data field reflecting the avaialble
space the producer can use.

By the way, because of this bug, it is possible to trigger a BUG_ON() when
we try to write the CRLF at the end of the chunk because the buffer is
full. It is unexpected. Only the stats applet may hit this bug.

To fix the issue, instead of writting this CRLF when the current chunk is
consumed, it is written before consuming the next one. This way, all space
reserved to create the chunk formatting is always placed before forwarding
data.

No backport needed.