Export TUN device statistics from the worker process.

When a worker process terminates in authenticated state, then
export statistics from the tun device (currently bytes_in and
bytes_out). These statistics are sent to main process using an
informational message just prior to process exit. The statistics
are also exported to the disconnect script using the STATS_BYTES_IN
and STATS_BYTES_OUT environment variables.

src/worker.h

@@ -202,6 +202,10 @@ typedef struct worker_st {
 	unsigned cert_auth_ok;
 	int tun_fd;
 
+	/* tun device stats */
+	uint64_t tun_bytes_in;
+	uint64_t tun_bytes_out;
+
 	/* information on the tun device addresses and network */
 	struct vpn_st vinfo;