System specific implementation for Ping event sources

[elinorbgr]

Currently Ping is implemented using a pipe, which is a generic unix way of implementing it.

However some platforms provide other primitives that could be used more efficiently for this, such as eventfd on Linux.

[notgull]

Some BSDs have EVFILT_USER, which can be used to wake up a kqueue without using a pipe. Similarly port_send can be used on Solaris. However I'm not sure how this would be exposed in polling.

[ids1024]

I see mio uses EVFILT_USER on kqueue platforms: https://github.com/tokio-rs/mio/blob/master/src/sys/unix/waker/kqueue.rs

https://docs.rs/mio/1.0.4/mio/struct.Waker.html#method.new allows creating a Waker that can be used on a particular Registry.