BUGFIX: don't try autocleanup inside system dirs

This safety feature addresses #2650 somewhat indirectly.
A list of system directories is defined.  When the uninstall
process encounters a "system directory" it does not touch that
directory at all
- no `chmod` on that dir
- no search for broken symlinks in that dir
- no cleanup of `.DS_Store` files in that dir
Explicitly listed files from a `pkg` receipt are not affected by
this change.  Individual files will still be removed, even if
those files are in a special "system directory"