MAJOR: proxy: enable abortonclose by default on HTTP proxies

MAJOR: proxy: enable abortonclose by default on HTTP proxies

As discussed on https://github.com/orgs/haproxy/discussions/3146 and on
the mailing list, there's a marked preference for having abortonclose
enabled by default when relevant. The point being that with todays'
internet, the large majority of requests sent with a closed input
channel are aborted requests, and that it's pointless to waste resources
processing them.

This patch now considers both "option abortonclose" and its opposite
"no option abortonclose" to figure whether abortonclose is enabled or
disabled in a backend. When neither are set (thus not even inherited
from a defaults section), then it considers the proxy's mode, and HTTP
mode implies abortonclose by default.

This may make some legacy services fail starting with 3.3. In this case
it will be sufficient to add "no option abortonclose" in either the
affected backend or the defaults section it derives from. But for
internet-facing proxies it's better to stay with the option enabled.