BUG/MEDIUM: mux-h1: Take care to update <kop> value during zero-copy forwarding

BUG/MEDIUM: mux-h1: Take care to update <kop> value during zero-copy forwarding

Since the extra field was removed from the HTX structure, a regression was
introduced when forwarding of chunked messages. The <kop> value was not
decreased as it should be when data were sent via the zero-copy
forwarding. Because of this bug, it was possible to announce a chunk size
larger than the chunk data sent.

To fix the bug, an helper function was added to properly update the <kop>
value when a chunk size is emitted. This function is now called when new
chunk is announced, including during zero-copy forwarding.

As a workaround, "tune.disable-zero-copy-forwarding" or just
"tune.h1.zero-copy-fwd-send off" can be set in the global section.

This patch should fix the issue #3230. It must be backported to 3.3.