write-out: add '%{proxy_used}'

Returns 1 if the previous transfer used a proxy, otherwise 0. Useful to
for example determine if a `NOPROXY` pattern matched the hostname or
not.

Extended test 970 and 972

src/tool_writeout.h

@@ -79,6 +79,7 @@ typedef enum {
   VAR_PRIMARY_IP,
   VAR_PRIMARY_PORT,
   VAR_PROXY_SSL_VERIFY_RESULT,
+  VAR_PROXY_USED,
   VAR_REDIRECT_COUNT,
   VAR_REDIRECT_TIME,
   VAR_REDIRECT_URL,