Add an explicit PROXY_FUZZ that just feeds a file into the backend.

This saves the need to fork and exec "cat", which should speed things
up.  It also ensures that the network output goes to /dev/null, which
should avoid problems with blocking when writing to a full pipe.

putty.h

@@ -291,7 +291,7 @@ enum {
      * Proxy types.
      */
     PROXY_NONE, PROXY_SOCKS4, PROXY_SOCKS5,
-    PROXY_HTTP, PROXY_TELNET, PROXY_CMD
+    PROXY_HTTP, PROXY_TELNET, PROXY_CMD, PROXY_FUZZ
 };
 
 enum {