us_timer_set uses wrong timings

[gotocoffee1]

In the epoll backend the conversion from milliseconds to nanoseconds is wrong. This means that times of less than one second do not work.
Instead it should be something like:

    struct itimerspec timer_spec = {
        {repeat_ms / 1000, repeat_ms * 1000000 % 1000000000},
        {ms / 1000, ms * 1000000 % 1000000000}
    };