Updates to trust anchoring

The unbound process needs write access to the directory where the trust anchor file exists as it writes the new file in the same dir and then preforms a move operation to ensure the update is atomic.  If the process does not have write permission you will see an error like the following

Dec  1 10:38:39 host unbound: [11768:0] error: could not open autotrust file for writing, /etc/unbound/root.anchor.11768-0: Permission denied

This merge does the following 
   * adds a new directory which is writable and stores the anchor file.  I considered changing the permissions of confdir however it felt as if it was downgrading the security of other files.
   * moves the anchor file to this new directory
   * initiate this file using unbound-anchor instead of a hardcoded DS record.
This is initiated using unbound-anchor in Exec['download-anchor-file']