A small set of changes to allow setting the hostname of the jail just
like we set hostnames of local nodes (v0.wa-jail-link.testbed.emulab.net). Also build a proper /etc/hosts file just like local nodes inside the jail. Minor change to dohostnames(); do not munge /etc/hosts in place! Generate a new copy and then atomically move into place. Phew, this was a close one! Mike has me really worried about race conditions now that I screwed the pooch in the frisbee client.