Start client backend through systemd

It'd be nice if this package did the grunt work to autostart urbackup, or at least, get it ready to be autostarted. I ended up making `/etc/systemd/system/urbackupclientbackend.service` based on the Debian .service file in the source repo:

```
[Unit]
Description=UrBackup Client backend
After=syslog.target network.target

[Service]
ExecStart=/usr/bin/urbackupclientbackend --config /etc/default/urbackupclient --no-consoletime
User=root

[Install]
WantedBy=multi-user.target
```

It'd also be cool if it had a good option for automatically starting the GUI. I don't know what the best way to accomplish that is, though.

Start by comparing the proposed /etc/systemd/system/urbackupclientbackend.service with the Debian .service file in the source repository and inspect how this AUR package installs files. Define how the client backend should be autostarted and whether GUI startup is in scope; done means the package provides a tested, appropriate startup configuration.

Written by the indexing model from the issue text.