MINOR: stream/mux: implement websocket stream flag

MINOR: stream/mux: implement websocket stream flag

Define a new stream flag SF_WEBSOCKET and a new cs flag CS_FL_WEBSOCKET.
The conn-stream flag is first set by h1/h2 muxes if the request is a
valid websocket upgrade. The flag is then converted to SF_WEBSOCKET on
the stream creation.

This will be useful to properly manage websocket streams in
connect_server().