New backend pieusb

Support for two Reflecta USB scanners:
 * - CrystalScan 7200 (model id 0x30)
 * - ProScan 7200 (model id 0x36)
(OEM is Pacific Image Electronics - PIE)

Backend by Jan Vleeshouwers based on discussions with
Michael Rickmann and Klaus Kaempf
Add pieusb to Makefile.am
pieusb_usb.c: Fix type in call to sanei_usb_read_bulk()
Delete generated files
sanei/Makefile.am: add sanei_ir.c
typedef SANE_Uint in sanei_ir.h, include it in pieusb.c
sane/sane.h: enable extended API definitions
dll.conf.in: enable pieusb
Move stiff.c to frontend
tstbackend: Add 'PIE' as recognized vendor
pieusb: Device number of Reflecta DigitDia 6000 is 0x3a/58
Prevent core, add debug
pieusb cleanup - compile .c files separately

plus many code cleanups and compiler warning fixes