Add a use-at-your-own-risk FreeBSD rc.d script for populating users

This is a use at your own risk shell script that can be dropped in /usr/local/etc/rc.d. It attempts to update accounts at boot time from the metadata server for FreeBSD-based GCE instances.

It has been tested on 10.0-RELEASE-p5 and 10-STABLE based builds.

For successful use, gce_logins_enable="YES" must be added to /etc/rc.conf.