rfe: option to install unbound

An option (maybe --with-unbound) to install the unbound caching resolver would be useful, particularly for mail servers.

In addition to installing the package, appending dns_server 127.0.0.1 to /etc/spamassassin/local.cf would configure spamassassin to use the local resolver. Possibly updating /etc/resolv.conf could be done.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information