Add (limited) support for mount requests from OCI config.json

[stgraber]

Currently we're completely ignoring the mount section from config.json.

That's obviously not exactly ideal as there may occasionally be reasonable requests in there which are actually required for a container :)

We should add logic to ignore all the stuff we mount ourselves (/dev, /proc, /sys, /sys/fs/cgroup, /dev/mqueue, /dev/pts, ...) but then for the stuff that we're not handling, effectively turn it into liblxc config so long as it's something that looks safe (would probably initially restrict to tmpfs).