Based on #280 by luca-barbieri

Currently run() results in the process waking up every second or every timer
tick if set lower because a timeout is passed to the OS wait function.

This changes the code so that run_once() still uses the timeout, but
run() will instruct the OS to wait forever when there are no timers.

The notify mechanism already uses a pipe for awakening the event loop,
so there should be no need to use a timeout for that.

This saves CPU time and battery, and is generally the correct behavior.

Miscellaneous notes:

 * We also fix epoll because the maximum timeout supported by the Linux kernel is
   i32::MAX, not isize::MAX.
 * We fix next_tick_ms() being truncated from u64 to usize
 * It requires carllerche/nix-rust#192 to allow specifying no timeout to kevent()
 * The public API of Poll::poll is changed, but that can be avoided if desired.

Closes #289