obfs4Listener.Accept() returns an error when obfs4 handshake fails, which in
practice happens routinely when someone scans a machine that has an open
listening port.
Currently the accept loop in server.go exits on first such error, which makes
further connections to the same port impossible.
This change wraps obfs4 handshake errors into a custom error type that
satisfies net.Error and presents itself as temporary, which will prevent such
errors from aborting the server.
