update clock at the beginning of seconds

Right now, the clock updates sometime in the middle of each second, by delaying ~1000ms from the last time it was updated. This causes extra system wakeups if another clock is running, looks bad, and misrepresents the computer's idea of what time it is.

Comparing to e.g. xclock -digital -update 1, you can see xclock updating before tint2's clock does.

This patch looks at the current time to decide when to register the next clock update, rather than using a recurring timer.