Allow non-privileged containers to create device nodes.

Allow non-privileged containers to create device nodes.

Such nodes could already be created by importing a tarball to a container; now
they can be created from within the container itself.

This gives non-privileged containers the mknod kernel capability, and modifies
their cgroup settings to allow creation of *any* node, not just whitelisted
ones.  Use of such nodes is still controlled by the existing cgroup whitelist.

Fixes #2191