BUG/MINOR: h2/h3: Only test number of trailers inserted in HTX message

BUG/MINOR: h2/h3: Only test number of trailers inserted in HTX message

When H2 or H3 trailers are inserted in an HTX message, we must take care to
not exceed the maximum number of trailers allowed in a message (same than
the maximum number of headers, i.e tune.http.maxhdr). However, all HTX
blocks in the HTX message were considered. Only TRAILERS HTX blocks must be
considered.

To fix the issue, in h2_make_htx_trailers(), we rely on the "idx" variable
at the end of the for loop. In h3_trailers_to_htx(), we rely on the
"hdr_idx" variable.

This patch must be backported to all stables versions for the H2 part and as
far as 2.8 for the H3 one.

pouet