BUG/MEDIUM: h2/hpack: fix emission of HPACK DTSU after settings change

BUG/MEDIUM: h2/hpack: fix emission of HPACK DTSU after settings change

Sadly, despite particular care, commit 39a0a1e12 ("MEDIUM: h2/hpack: emit
a Dynamic Table Size Update after settings change") broke H2 when sending
DTSU. A missing negation on the flag caused the DTSU_EMITTED flag to be
lost and the DTSU to be sent again on the next stream, and possibly to
break flow control or a few other internal states.

This will have to be backported wherever the patch above was backported.

Thanks to Yves Lafon for notifying us with elements to reproduce the
issue!

(cherry picked from commit c7d85485a00bd9862ecb726ad1242c2ba724a8ca)
Signed-off-by: Willy Tarreau <w@1wt.eu>