BUG/MINOR: mux-h1: Send a 400-bad-request on shutdown before the first request

BUG/MINOR: mux-h1: Send a 400-bad-request on shutdown before the first request

Except if we must silently ignore empty connections by enabling
http-ignore-probes or dontlognull options, when a client connection is
closed before the first request, a 400-bad-request response must be sent
with the corresponding log message. However, that is broken since the commit
fc473a6453 ("MEDIUM: mux-h1: Rely on the H1C to deal with shutdown for
reads").

The bug is subtle. Parsing errors are no longer reported on connection errors
before the first request while it should be.

This patch must be backported where the above commit is (as far as 2.7).