BUG/MEDIUM: h3: Properly limit the number of headers received

BUG/MEDIUM: h3: Properly limit the number of headers received

The number of headers are limited before the decoding but pseudo headers and
cookie headers consume extra slots. In practice, this lowers the maximum number
of headers that can be received.

To workaround this issue, the limit is doubled during the frame decoding to be
sure to have enough extra slots. And the number of headers is tested against the
configured limit after the HTX message was created to be able to report an
error. Unfortunatly no parsing error are reported because the QUIC multiplexer
is not able to do so for now.

The same is performed on trailers to be consistent with H2.

This patch should be backported as far as 2.6.