sane-airscan integration

Please, consider replacement of sane-escl with sane-airscan:

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

sane-airscan implements many features, currently missed in sane-escl. Most notable are

• Full ADF support (simplex/duplex/multipage)
• Working sane_cancel() support
• Working sane_get_select_fd() support
• Very well instrumented for troubleshooting (it can write to file full protocol trace, including received images)
• Works correctly if called from multithreaded program. sane_cancel() can be called from signal handler

Planned features are:

• Support PDF as a transport format, for 1-bit B&W scan
• Let sane_read() to return as much as possible image data while image is still being downloaded from scanner, to improve user experience
• May be, WSD protocol, if there is a noticeable amount of devices implementing WSD but missing eSCL

It has a constantly growing list of devices known to work with

sane-airscan is written with security in mind. Data, received from untrusted source, always carefully checked for consistency and never gets saved into the fixed-size buffer.

sane-airscan does its best to leave scanner in a consistent state, even in a case of scan errors or API misuse

It builds without warnings on 15 Linux distros (with -W -Wall -Werror gcc options)