Initial commit, add README.md to describe the intent of varball

The idea is to enable keeping /var off physical media while the system is
running, but to archive it on shutdown and extract it again on boot so that
the persistence normally afforded /var is not lost.
Add etc/rc.d/varball

As intended, this rc(8) script extracts a tar file (/var.tar by default) to
/var on boot, and archives /var to the same tar file on shutdown.