MEDIIM: mux-h1: Add splicing support for chunked messages

MEDIIM: mux-h1: Add splicing support for chunked messages

When the HTX was introduced, we have lost the support for the kernel
splicing for chunked messages. Thanks to this patch set, it is possible
again. Of course, we still need to keep the H1 parser synchronized. Thus
only the chunk content can be spliced. We still need to read the chunk
envelope using a buffer.

There is no reason to backport this feature. But, just in case, this patch
depends on following patches:

  * "MEDIUM: filters/htx: Don't rely on HTX extra field if payload is filtered"
  * "MINOR: mux-h1: Add function to prepend the chunk crlf to the output buffer"
  * "MINOR: mux-h1: Add function to append the chunk size to the output buffer"
  * "REORG: mux-h1: Rename functions to emit chunk size/crlf in the output buffer"
  * "MEDIUM: mux-h1: Split h1_process_mux() to make code more readable"