MEDIUM: h3: rely on the generic trailers parser

MEDIUM: h3: rely on the generic trailers parser

h3_trailers_to_htx() performed exactly the same controls as the generic
http_trailers_to_htx() parser, but was relying on traces and glitches
for each failure case that was codified on the latter parser.

Let's just drop the parsing loop for a direct call to the function and
a check on the return codes to decide what trace/glitch to emit.

It's worth noting that H3_ERR_MESSAGE_ERROR is not set when failing to
add a header, not sure whether this is a design decision or an overlook.
In any case this behavior was preserved.

Note that pseudo-headers are detected on their syntax here, while h2
used to detect them on their assigned number, so this remains a tiny
difference between the two parsers at this point.

Regtests continue to pass.