lxc-start-unconfined softlink can go bad

when sharing a /var/lib/docker dir with more than one distribution, an existing lxc-start-unconfined softlink may point to a non-existant path, following that link (as Stat does) will cause the daemon to fail to start

for eg, I'm swapping between my original ubuntu install and booting from a boot2docker sd-card (finally found a use for my 10 year old 32Meg SD cards!!)

boot2docker has /bin/lxc-startup, ubuntu has /usr/bin/lxc-startup