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

config/doc.go

@@ -715,6 +715,11 @@ describe-static" and "mox config describe-domains":
 				# unmodified. New X-Forwarded-{For,Host,Proto} headers are set. Any query string
 				# in the URL is ignored. Requests are made using Go's net/http.DefaultTransport
 				# that takes environment variables HTTP_PROXY and HTTPS_PROXY into account.
+				# Websocket connections are forwarded and data is copied between client and
+				# backend without looking at the framing. The websocket 'version' and
+				# 'key'/'accept' headers are verified during the handshake, but other websocket
+				# headers, including 'origin', 'protocol' and 'extensions' headers, are not
+				# inspected and the backend is responsible for verifying/interpreting them.
 				URL:
 
 				# Headers to add to the response. Useful for adding security- and cache-related