Revert "experimental/shadow"

This reverts commit 788e760b.  NixOS
doesn't even have an /etc/skel.  And if we had, 1) NixOS has the
ability to create regular files rather than symlinks in /etc
(environment.etc.<file>.mode = ...), so files in /etc/skel that should
be copied as regular files can be supported; and 2) we may actually
*want* to copy a symlink.

Also, typos in file names.  Bleh.