add reverse proxying websocket connections

if we recognize that a request for a WebForward is trying to turn the
connection into a websocket, we forward it to the backend and check if the
backend understands the websocket request. if so, we pass back the upgrade
response and get out of the way, copying bytes between the two. we do log the
total amount of bytes read from the client and written to the client. if the
backend doesn't respond with a websocke response, or an invalid one, we respond
with a regular non-websocket response. and we log details about the failed
connection, should help with debugging and any bug reports.

we don't try to parse the websocket framing, that's between the client and the
backend.  we could try to parse it, in part to protect the backend from bad
frames, but it would be a lot of work and could be brittle in the face of
extensions.

this doesn't yet handle websocket connections when a http proxy is configured.
we'll implement it when someone needs it. we do recognize it and fail the
connection.

for issue #25

rfc/index.md

@@ -297,6 +297,16 @@ and many more, see http://sieve.info/documents
 9157	Revised IANA Considerations for DNSSEC
 9276	Guidance for NSEC3 Parameter Settings
 
+# HTTP
+
+2616	Hypertext Transfer Protocol -- HTTP/1.1
+7230	Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
+9110	HTTP Semantics
+
+# Websockets
+
+6455	The WebSocket Protocol
+
 # More
 
 3339	Date and Time on the Internet: Timestamps