MINOR: http-htx: pass extra flags to the HTTP trailers parser

MINOR: http-htx: pass extra flags to the HTTP trailers parser

Let's introduce HTTP parser flags (HTTP_PF_*) and rely on them for the
trailers parser. This one will need them to decide to enable upper case
characters or not in header names in order to reuse it for H1 parsing.

For now we only pass the direction. The current parser doesn't use it
but the goal is to generalize the mechanism to other parsers in the
future.