Add a setup script for unbound(8) called local-unbound-setup.  It
generates a configuration suitable for running unbound as a caching
forwarding resolver, and configures resolvconf(8) to update unbound's
list of forwarders in addition to /etc/resolv.conf.  The initial list
is taken from the existing resolv.conf, which is rewritten to point to
localhost.  Alternatively, a list of forwarders can be provided on the
command line.

To assist this script, add an rc.subr command called "enabled" which
does nothing except return 0 if the service is enabled and 1 if it is
not, without going through the usual checks.  We should consider doing
the same for "status", which is currently pointless.

Add an rc script for unbound, called local_unbound.  If there is no
configuration file, the rc script runs local-unbound-setup to generate
one.

Note that these scripts place the unbound configuration files in
/var/unbound rather than /etc/unbound.  This is necessary so that
unbound can reload its configuration while chrooted.  We should
probably provide symlinks in /etc.

Approved by:	re (blanket)