#305 bt-remove-unselected-file doesn't remove dirs

[curaga]

Using aria2 1.17.0, the remove-unselected-file feature fails to remove unselected dirs.

Example:

Torrent has this layout:
a/1
a/2
b/1
b/2

If you select only file b/1, that's the only file that exists after download - but the directory a also still exists.

[tujikawa]

Currently aria2 only deletes files and not parent directories. Need to do some traversal to delete directory up to the directory specified by -d option.