MINOR: mux-h1: Add a flag to ignore the request payload

MINOR: mux-h1: Add a flag to ignore the request payload

There was a flag to skip the response payload on output, if any, by stating
it is bodyless. It is used for responses to HEAD requests or for 204/304
responses. This allow rewrites during analysis. For instance a HEAD request
can be rewrite to a GET request for any reason (ie, a server not supporting
HEAD requests). In this case, the server will send a response with a
payload. On frontend side, the payload will be skipped and a valid response
(without payload) will be sent to the client.

With this patch we introduce the corresponding flag for the request. It will
be used to skip the request payload. In addition, when payload must be
skipped for a request or a response, The zero-copy data forwarding is now
disabled.