Scanner not usable without CUPS

[Cigaes]

libsane-hpaio relies on CUPS to detect networked all-in-one scanner-printers. That means that if CUPS is not running, networked devices are usable by specifying the URL, but they will not be detected. Asa consequence, they will not appear in GUI device selection dialogs, and not be usable from software that does not allow to specify an URL.

Note: since CUPS is rather heavy, not always running it is a legitimate use case, especially on small hardware, and especially for people like me who scan much more frequently than we print.

libsane-hpaio should provide a way to list available networked scanner-printers without relying on CUPS. Possibly `/etc/sane.d/hpaio.conf` containing lines like:

```
hpaio:/net/deskjet_3050a_j611_series?ip=10.0.1.155
hpaio:/net/Photosmart_2570_series?ip=10.0.0.32
```

Note: I have not run hp-doctor, I have read the source code in `scan/sane/hpaio.c`, especially the functions `GetCupsPrinters()` and `DevDiscovery()`.

Thanks.