directory, then a file with that symlink as a prefix can drop a file
outside of the current directory, which can be a security hole.
Plug this hole by refusing to extract files if a prefix of the
pathname is a symlink. The -P option disables this check.
table for the hardlink cache. This dramatically improves
performance when archiving millions of hardlinked files.
While I'm here, clean up some style bugs (per Bruce Evans)
and clarify some comments.
