Perform minute alignment for refreshes only once per minute

Long description:

This patch ensures that the minute alignment for refreshes is only
done once per minute to make sure that the refresh interval time
is obeyed as much as possible after a refresh has been forced from
the outside. This also reduces the total number of refreshs in such
cases.

To apply this patch, use:
curl http://cr.i3wm.org/patch/465/raw.patch | git am

@@ -677,7 +677,12 @@ int main(int argc, char *argv[]) {

                  * that we start with :00 on every new minute. */

                 struct timeval current_timeval;

                 gettimeofday(&current_timeval, NULL);

-                struct timespec ts = {interval - 1 - (current_timeval.tv_sec % interval), (10e5 - current_timeval.tv_usec) * 1000};

+                /* how many seconds until the next full minute? */

+                int gap = 60 - (current_timeval.tv_sec % 60);

+                struct timespec ts = {

+                        interval - 1 - (interval >= gap ? current_timeval.tv_sec % interval : 0),

+                        (10e5 - current_timeval.tv_usec) * 1000

+                };

                 nanosleep(&ts, NULL);

         }

 }