MINOR: mux-h1: Add a flag on H1 streams with a response known to be bodyless

MINOR: mux-h1: Add a flag on H1 streams with a response known to be bodyless

In HTTP/1, responses to HEAD requests and 204/304 must not have payload. The
H1S_F_BODYLESS_RESP flag is not set on streams that should handle such
responses, on the client side and the server side.

On the client side, this flag is set when a HEAD request is parsed and when
a 204/304 response is emitted. On the server side, this happends when a HEAD
request is emitted or a 204/304 response is parsed.