Previously, the main proxy loops in `run_server_child` and
`run_client_child` alternated between making transfers from their shared
channel to their individual wayland compositor/application channels.

By using socket connections in nonblocking mode, the proxy now can
simultaneously maintain transfers in both directions. This ensures that
large transfers (such as a full-screen image) do not prevent e.g. key
release events from being transferred.

Additionally, as the two proxy loops were very similar, they have been
 combined into the `main_interface_loop` routine.

The code still does not correctly handle the case where a read from
the wayland compositor/application fills the entire read buffer and
truncates a sent message, but it is now easier to adjust for that case.