Cleanup the usage of ScanDirection: use the symbolic names for the

possible ScanDirection alternatives rather than magic numbers
(-1, 0, 1).  Also, use the ScanDirection macros in a few places
rather than directly checking whether `dir == ForwardScanDirection'
and the like. Per patch from James William Pye. His patch also
changed ScanDirection to be a "char" rather than an enum, which
I haven't applied.