sane-backends: run (xinetd) saned as non-root

sane-backends: run (xinetd) saned as non-root

saned requires write access scanner USB bus for its process and
to usblp/bind in order to rebind multifunctional printers back
to usblp (for printing with p910nd).

A hotplug script monitor new USB devices for scanners. Scanners
are detected by searching /usr/share/sane/<vendor>-<backend>.usbid
for the device product_id.

The package saned creates user saned:scanners. Access is granted to
group scanners.

The default xinetd conf was updated to run as saned:scanner.

sane-daemon pkg now has a postinst trigger that runs udevtrigger for
granting perms where there are connected scanners during installation.

Existing hotplug scripts from hplip were removed. They were mostly
useless.

Signed-off-by: Luiz Angelo Daros de Luca <redacted>