fixes the broken "ADF empty" detection on the Fujitsu scanners
when connected via USB. It also fixes a sense-request issues,
which were required in order to determine whether the ADF is
empty. It also eliminates some duplicate code blocks.
renamed some functions
- several bugfixes
- supports the M3097
- get threshold, contrast and brightness from vpd
- imprinter support
- get_hardware_status now works before calling sane_start
- avoid unnecessary reload of options when using source=fb