Convert into edge-triggered model

Created by: jD91mZM2

This will allow pseudo-terminals to work cleanly with tokio.

Changes:

• It now returns EAGAIN instead of Ok(0) on a failed nonblocking read.
• It now sends write events when registered to notify that writing is always available.
• It no longer sends events over and over.

Warning: This BREAKS the following projects:

• userutils' getty (fork)
• orbterm (fork)

I am ready to send pull requests that fixes both of the above.