add -o/--output option

src/include/options.h

@@ -54,11 +54,12 @@ struct opts_s {           /* structure describing run-time options */
 	bool stop_at_size;             /* set if we stop at "size" bytes */
 	bool sync_after_write;         /* set if we sync after every write */
 	bool direct_io;                /* set if O_DIRECT is to be used */
-	bool discard_input;            /* set to write nothing to stdout */
+	bool discard_input;            /* set to write nothing to output */
 	double interval;               /* interval between updates */
 	double delay_start;            /* delay before first display */
 	pid_t watch_pid;	       /* process to watch fds of */
 	int watch_fd;		       /* fd to watch */
+	/*@keep@*/ /*@null@*/ char *output; /* fd to write output to */
 	unsigned int average_rate_window; /* time window in seconds for average rate calculations */
 	unsigned int width;            /* screen width */
 	unsigned int height;           /* screen height */