systemctl: also use chase_symlinks for dropins

systemctl: also use chase_symlinks for dropins

The general rule is:
- code in shared/ should take an "original_root" argument (possibly NULL)
  and pass it along down to chase_symlinks
- code in core/ should always use specify original_root==NULL, since we
  don't support running the manager from non-root directory
- code in systemctl and other tools should pass arg_root.

For any code that is called from tools which support --root, chase_symlinks
must be used to look up paths.