Don't extract anything if we are not fully mounted

This way if you extract something to / or /var it doesn't fail due
to a readonly filesystem.
Update all tardirs if onetime.tgz was extracted

If someone put something in /var in their onetime.tgz it would normally
just go away on reboot.  Instead update it.
Allow for a /flashrd.site script in the onetime.tgz

This gives a hook at this point to adjust things as you see fit.  In
my case I adapt a "standard" siteXX.tgz with the install.site file by
moving /install.site to /etc/rc.firsttime.