Alter ltfs_check_medium to do a more thorough detection of the state of
the partitions after the detected indices and set the eod/append
position appropriately if the partition ends in data + filemark

Fixes LinearTapeFileSystem#272

Change parameter list to include vol and partid instead of device and part
number and emit 11020E or 11023E on tape_seek failure as appropriate for the
given partid

If a partition ends in in a potential partial index after the known consistent
index on that partition, only truncate it if the parameter 'fix' is true in
ltfs_check_medium()