MEDIUM: mux-h1: Support zero-copy forwarding for chunks with an unknown size

MEDIUM: mux-h1: Support zero-copy forwarding for chunks with an unknown size

Till now, for chunked messages, the H1 mux used the size requested during
the zero-copy forwarding negotiation as the chunk size. And till now, this
was accurate because the requested size was indeed the chunk size on the
producer side.

But this will be a problem to implement the zero-copy forwarding on some
applets because the content size is not known during the nego but only when
it is produced. Thanks to previous patches, it is now possible to know the
requested size is not exact and we are able to reserve a larger space to
write the chunk size later, in h1_done_ff(), with some padding.