MSDOS time/date was read in wrong order and also did not take into account
that the bit ranges in the shortis are in little-endian.
Remodel modification_time/date to be one struct with fat_time, fat_date LE shorts
and then synthetic values for day, hours, minute etc and also a unix field with the
timestamp as unix time.
Also refactor and clenaup extra fields/extended code a bit.
Fixes#792