removeDirectoryRecursive and removePathForcibly are supposed to never
traverse symbolic links, which would avoid unintended deletion of files
outside the specified directory tree.

The previous implementation did not "lock onto" the directory while
enumerating its entries, allowing concurrent processes to replace the
directory with a symlink right before or during the enumeration and
tricking the deleter into traversing the symlink (#97).

This commit mitigates the issue on POSIX systems by acquiring file
descriptors to every directory it traverses.