Whiteout detection should be more robust

[mgree]

Right now we detect whiteout files with [ -c "$changed_file" ] && ! [ -s "$changed_file" ]... but we should also check that the major/minor numbers are 0/0 in case someone has some non-whiteout, empty character device file. (Hard to imagine this happening, but safety first.)

[angelhof]

This is resolved by #71. However, ideally we also want to add a test that checks that non-whiteout character device files created in try are not mistakenly assumed to be whiteout files. This is not that easy to do since it either requires sudo access (and mknod still fails in try due to permissions), or a fuse character device file.

Let's keep this issue open until such a test is implemented. Some discussion on attempts to implement this test can be found in #71.