Remove non-existing files, add stop, use synctl

Non-existing files, when running the suggested from https://github.com/matrix-org/synapse#configuring-synapse
/etc/synapse/log_config.yaml so the --log-config leads to an error
/etc/sysconfig/synapse The environment-file or even the /etc/sysconfig does not exist in arch linux

Also instead of calling python2 we use synctl, as this seems to be the proper way to start it, and it gives us a more useful error in the systemctl status. And we now allow stop (and therefore restart).