Cleanup mft file handling, especially the stale mft bits.

Cleanup mft file handling, especially the stale mft bits.
Move staleness check up into mft_parse_econtent() to simplify code.
Remove the big FIXME bits since they are no longer needed. The parent
process will only process MFTs that are not stale.
Cleanup a few other bits mainly unneccessary else if cascades and
use valid_filename() to check if the filename embedded in the mft
fileandhash is sensible.
OK tb@