It was way too ambiguous who's the source and who's the destination, and it
didn't really follow a logical pattern. "Local port" vs "Peer port" is super
obvious, so let's call it that.
Make the Socket functions take a FileDescriptor& rather than a socket role
throughout the code. Also change threads to block on a FileDescriptor,
rather than either an fd index or a Socket.
If connect() is called on a non-blocking socket, it will "fail" immediately
with -EINPROGRESS. After that, you select() on the socket and wait for it to
become writable.