Convert enrollment function to return a signed integer, which is negative
on error or corresponds into fp_enroll_result otherwise.
Now we can treat a 'FAIL' condition differently from an actual error: fail
means that enrollment didn't complete because the data was nonsense or
whatever (e.g. scanned a different finger for each stage?).
Updated upekts accordingly.
upekts will need to know when the first enrollment stage is attempted for
the first time, so add an __enroll_stage counter which actually indicates
the next stage to enroll. -1 is a special value and it means 0 is next *and*
it is the initial attemt.
Added more debug output to the enroll handler.
Added new fp_enroll_status codes for too short or uncentered scans.
Changed the print_data allocator to consider the device rather than the
driver, this feels more natural. Added missing return value.
Make fp_enroll_status codes start at 1. 0 can now be used as a
special/temporary value by the drivers. Also check that we aren't exceeding
the number of enroll stages.
Also add a missing exit() call to the verify example and update for the
above.
