Fix kqueue on platforms where C's long is i32

src/sys/unix/kqueue.rs

@@ -74,7 +74,11 @@ impl Selector {
     ) -> io::Result<bool> {
         let timeout = timeout.map(|to| libc::timespec {
             tv_sec: cmp::min(to.as_secs(), time_t::max_value() as u64) as time_t,
-            tv_nsec: libc::c_long::from(to.subsec_nanos()),
+            // `Duration::subsec_nanos` is guaranteed to be less than one
+            // billion (the number of nanoseconds in a second), making the
+            // cast to i32 safe. The cast itself is needed for platforms
+            // where C's long is only 32 bits.
+            tv_nsec: libc::c_long::from(to.subsec_nanos() as i32),
         });
         let timeout = timeout
             .as_ref()