MINOR: proto_htx: Use full HTX messages to send 401 and 407 responses

MINOR: proto_htx: Use full HTX messages to send 401 and 407 responses

Instead of replying by adding an OOB block in the HTX structure, we now add a
valid HTX message. The old code relied on the function http_reply_and_close() to
send 401/407 responses. Now, we push it in the response's buffer. So we take
care to drain the request's channel and to shutdown the response's channel for
the read.