systemd: Increase backlog for unix sockets

Use the default backlog size, SOMAXCONN (128), which is the same as for
the unix socket in module-protocol-native. If the backlog queue gets
full incoming connections will be closed before they can be accepted.
This causes connect() to fail with EAGAIN on the client side and since
the server socket is closed it will not be possible to use the client
socket further.