REGTESTS: http-messaging: test the option httpclose

REGTESTS: http-messaging: test the option httpclose

The option only appeared in other tests as a convenient way to get rid
of the idle connections, its own behavior was never checked, and
"option http-pretend-keepalive", which modifies it, was not covered
either.

The five proxies here cover the four ways it can be declared. The side
whose connection is closed is the one the option is declared on: the
client one for a frontend, the server one for a backend, both for a
listener, and the "no option" form undoes what a defaults section had
set. The header it adds in that direction is checked by the server for
the request side and by the client for the response side.

Whether the client connection was closed is directly observable, and
whether the server one was is reported by bc_reused, since a connection
which was closed cannot be reused by the next request: the frontend
case reuses its server connection while the listener case cannot. This
is also how "http-pretend-keepalive" is checked, the server not seeing
the header while its connection is closed all the same.