MEDIUM: mux-h2: Use RAW_DATA blocks to store tunneled data

MEDIUM: mux-h2: Use RAW_DATA blocks to store tunneled data

For a tunneled H2 stream, the end of the HTTP message is reported just after
the headers, when H2_SF_BODY_TUNNEL flag is set. So any DATA frame received
after that carries tunneled data and must be stored in a RAW_DATA block.
h2_frt_transfer_data() was updated accordingly.

Thanks to this change, HTTP data and tunneled data are no longer mixed on
the H2 side. The early end of the message is thus preserved when tunneled
data are received, and the next commit is able to remove the workaround that
was necessary to keep both in separate buffers.