Add AsyncFd::try_io

[jyn514]

Is your feature request related to a problem? Please describe.

I have a Unix socket where I want to call libc::sendmsg on the file descriptor directly. Right now, I have to use

let guard = async_fd.writable().await;
sendmsg(...);
guard.clear_ready();

which is error prone.

Describe the solution you'd like

It would be convenient to allow using try_io instead, like for UdpSocket:

async_fd.try_io(Interest::WRITABLE, || sendmsg(...));

Describe alternatives you've considered
Keep using writable() guard.
Implement sendmsg APIs in tokio directly (#2975).

Additional context
https://discord.com/channels/500028886025895936/500336333500448798/979801318375964733

[Noah-Kennedy]

This seems like a good idea.

[Pyrromanis]

I'd like to give this a try

[Noah-Kennedy]

@jyn514 we have unix socket types, why not use those for your use case?

[jyn514]

@Noah-Kennedy I have a trait that abstracts over different socket types.

[Noah-Kennedy]

Ah ok