export: fix systemd dependencies

With the previous way systemd services were exported, you had to enable every single process one by one to get the service operational.

Consider the following Procfile:

    web: bundle exec thin start
    worker: bundle exec worker start

Exported with:

    foreman export systemd /usr/lib/systemd/system -a myapp -c all=1,worker=3

You will have to perform the following to start the service:

    systemctl enable myapp-web-1.service
    systemctl enable myapp-worker-1.service
    systemctl enable myapp-worker-2.service
    systemctl enable myapp-worker-3.service
    systemctl enable myapp-web.target
    systemctl enable myapp-worker.target
    systemctl start myapp.target

Thats 7 commands, and you have to know the names of each service. Nasty.

With the changes here, you will have to perform the following:

    systemctl start myapp.target

*(with the previous code, you couldn't even start `myapp.target` without first enabling everything, which may not be desired)*

And to enable it on boot:

    systemctl enable myapp.target

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Additionally, this change also adds the dependency to 'multi-user.target' (the default behavior for all services). This will properly shut down (or start) the app when the system changes runlevels. The previous method did not do this.