BUG/MINOR: mux-h2: Properly handle full or truncated HTX messages on shut

BUG/MINOR: mux-h2: Properly handle full or truncated HTX messages on shut

On shut, truncated HTX messages were not properly handled by the H2
multiplexer. Depending on how data were emitted, a chunked HTX message
without the 0-CRLF could be considered as full and an empty data with ES
flag set could be emitted instead of a RST_STREAM(CANCEL) frame.

In the H2 multiplexer, when a shut is performed, an HTX message is
considered as truncated if more HTX data are still expected. It is based on
the presence or not of the H2_SF_MORE_HTX_DATA flag on the H2 stream.
However, this flag is set or unset depending on the HTX extra field
value. This field is used to state how much data that must still be
transferred, based on the announced data length. For a message with a
content-length, this assumption is valid. But for a chunked message, it is
not true. Only the length of the current chunk is announced. So we cannot
rely on this field in that case to know if a message is full or not.

Instead, we must rely on the HTX start-line flags to know if more HTX data
are expected or not. If the xfer length is known (the HTX_SL_F_XFER_LEN flag
is set on the HTX start-line), it means that more data are always expected,
until the end of message is reached (the HTX_FL_EOM flag is set on the HTX
message). This is true for bodyless message because the end of message is
reported with the end of headers. This is also true for tunneled messages
because the end of message is received before switching the H2 stream in
tunnel mode.

This patch must be backported as far as 2.8.

(cherry picked from commit b70921f2c19a0612a4f1e31ef16cf8c8b216fdf9)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 5341a464d733e7a562f5de3b94343ef027f8f079)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>