Remove PROXY_CHANGE_{SENT,CLOSING,ACCEPTING}.

These were just boilerplate in all the proxy negotiation functions:
every negotiator had to contain a handler for each of these events,
and they all handled them in exactly the same way. Remove them and
centralise the handling in the shared code.

A long time ago, some of these event codes were added with purpose in
mind. PROXY_CHANGE_CLOSING was there to anticipate the possibility
that you might need to make multiple TCP connections to the proxy
server (e.g. retrying with different authentication) before
successfully getting a connection you could use to talk to the
ultimate destination. And PROXY_CHANGE_ACCEPTING was there so that we
could use the listening side of SOCKS (where you ask the proxy to open
a listening socket on your behalf). But neither of them has ever been
used, and now that the code has evolved, I think probably if we do
ever need to do either of those things then they'll want to be done
differently.

proxy/proxy.h

@@ -39,10 +39,7 @@ struct ProxySocket {
                   */
 
 #define PROXY_CHANGE_NEW      -1
-#define PROXY_CHANGE_CLOSING   0
-#define PROXY_CHANGE_SENT      1
 #define PROXY_CHANGE_RECEIVE   2
-#define PROXY_CHANGE_ACCEPTING 3
 
     /* something has changed (a call from the sub socket
      * layer into our Proxy Plug layer, or we were just
@@ -57,19 +54,11 @@ struct ProxySocket {
      * (for use by proxy's negotiate function)
      */
 
-    /* closing */
-    PlugCloseType closing_type;
-    const char *closing_error_msg;
-
     /* receive */
     bool receive_urgent;
     const char *receive_data;
     int receive_len;
 
-    /* accepting */
-    accept_fn_t accepting_constructor;
-    accept_ctx_t accepting_ctx;
-
     /* configuration, used to look up proxy settings */
     Conf *conf;