fix(bug): ns->us overflow handling

[HalFrgrd]

I ran into an odd bug on macos. If my code polls for just under 5s (say 4.999_999_123 seconds), I immediately receive an EINVAL error.

I believe this is due to the rounding logic. With the current logic, we set tv_usec to 1_000_000 when the nanosecond remainder is >= 999_999_001. This creates an invalid timeval on macOS and the OS returns EINVAL.

This is a small example that I need to test on macOS:

  use rustix::event::{fd_set_insert, fd_set_num_elements, select, FdSetElement, Timespec};         
  use rustix::fd::AsRawFd;
  use rustix::pipe::pipe;

  fn main() {
      // Create a pipe so we have a valid file descriptor to pass to select()
      let (reader, _writer) = pipe().unwrap();
      let nfds = reader.as_raw_fd() + 1;
      let num_elems = fd_set_num_elements(1, nfds);

      // 1. 999_999_000 nanoseconds -> (999_999_000 + 999)/1000 = 999_999 μs (< 1,000,000 μs)      
      let mut readfds1 = vec![FdSetElement::default(); num_elems];
      fd_set_insert(&mut readfds1, &reader);
      let res1 = unsafe {
          select(
              nfds,
              Some(&mut readfds1),
              None,
              None,
              Some(&Timespec {
                  tv_sec: 0,
                  tv_nsec: 999_999_000,
              }),
          )
      };
      println!("999_999_000 ns result: {:?}", res1);

      // 2. 999_999_123 nanoseconds -> (999_999_123 + 999)/1000 = 1_000_000 μs
      let mut readfds2 = vec![FdSetElement::default(); num_elems];
      fd_set_insert(&mut readfds2, &reader);
      let res2 = unsafe {
          select(
              nfds,
              Some(&mut readfds2),
              None,
              None,
              Some(&Timespec {
                  tv_sec: 0,
                  tv_nsec: 999_999_123,
              }),
          )
      };
      println!("999_999_123 ns result: {:?}", res2); // Should show an error on macos.
  }