Improve statschannel HTTP Connection: header protocol conformance

Improve statschannel HTTP Connection: header protocol conformance

In HTTP/1.0 and HTTP/1.1, RFC 9112 section 9.6 says the last response
in a connection should include a `Connection: close` header, but the
statschannel server omitted it.

In an HTTP/1.0 response, the statschannel server can sometimes send a
`Connection: keep-alive` header when it is about to close the
connection. There are two ways:

If the first request on a connection is keep-alive and the second
request is not, then _both_ responses have `Connection: keep-alive`
but the connection is (correctly) closed after the second response.

If a single request contains

	Connection: close
	Connection: keep-alive

then RFC 9112 section 9.3 says the keep-alive header is ignored, but
the statschannel sends a spurious keep-alive in its response, though
it correctly closes the connection.

To fix these bugs, make it more clear that the `httpd->flags` are part
of the per-request-response state. The Connection: flags are now
described in terms of the effect they have instead of what causes them
to be set.