libwebsockets

h2: post buflist: track rx_content_length

On h2 server POST, there's a race to see if the POST body is going to be
received coalesced with the headers.

The problem is on h2, we can't action the stream http request or body until
the stream is writeable, since we may start issuing the response right away;
there's already DEFERRING_ACTION state to manage this.  And indeed, the
coalesced, not-immediately-actionable POST body is buflisted properly.

However when we come to action the POST using buflisted data, we don't follow
the same pattern as dealing with the incoming data immediately.

This patch aligns the pattern dumping the buflist content to track
expected rx_content_length and handle BODY_COMPLETION if we got to
the end of it, along with removal from the pt list of wsi with pending
buflists if we used it up.