Implement From<EventFd> for OwnedFd

[nekto0n]

User story

In some cases a user might need to send an event FD to a remote side via UNIX socket (in our case - over a DBus connection) and currently there's no way to do that with event FD except for .duping to get an owned FD and then dropping the original.

Solution

Pretty much explained in the code comments:

• we need to get the Inner first to deregister the FD from the tokio event loop
• then do a trick with std::mem::forget to avoid closing the FD

Tests

Tested that it works in my project 🤷

[nekto0n]

@polachok, hi! Could you please take a look?