SANE backend for AirScan (eSCL) and WSD document scanner

Similar to how most modern network printers support "driverless" printing,
using the universal vendor-neutral printing protocol, many modern network
scanners and MFPs support "driverless" scanning.

Driverless scanning comes in two flavors:

* Apple AirScan or AirPrint scanning (official protocol name is eSCL)
* Microsoft WSD, or WS-Scan (term WSD means "Web Services for Devices)

This backend implements both protocols, choosing automatically between them.
It was successfully tested with many devices from Brother, Canon, Dell,
Kyocera, Lexmark, Epson, HP, OKI, Panasonic, Pantum, Ricoh, Samsung and
Xerox both in WSD and eSCL modes.

WWW: https://github.com/alexpevzner/sane-airscan
