install: Add systemd support

Change the way you run it so that instead of running ./install directly,
you choose the relevant init system version and run that.
Add blank sample config for now

We can fill it in later, but it needs to exist as the installer wants to install it.
install: Be more selective about what we wipe when re-installing

We want to completely replace code directories (as opposed to a simple overwrite,
this won't cause problems when a file is no longer present or renamed).
We don't want to replace the config file.
We don't want to delete other stuff in the existing install dir, eg. auction logs.
We don't want to re-install dependencies unless we have to.