Fix build on FreeBSD

[wezm]

This PR allows tokio (4d514b7) to compile on FreeBSD again. Prior to this change the following error was encountered:

error[E0425]: cannot find function `hup` in this scope
   --> src/reactor/poll_evented.rs:443:9
    |
443 |         hup() | UnixReady::aio() | UnixReady::lio()
    |         ^^^ not found in this scope

This is on FreeBSD 11.1-RELEASE-p6, rustc 1.24.0.

I had a hard time working out where this hup function was supposed to be coming from. I came up with the following changes, which compile and the test suite passes. I fully admit I haven't studied the code enough to really know what is supposed to be happening though. Additionally I haven't tested the DragonflyBSD change but assume it's also in the same camp. Further review appreciated.

[wezm]

Fixes #219

[carllerche]

@wezm don't worry about a screenshot.

[carllerche]

@wezm it looks like the all() function isn't used anymore? Perhaps it can just be deleted?

[wezm]

@carllerche You're right. I've removed it and all seems well.

[carllerche]

Thanks!