Support creation and use of lxc-cirros by unprivileged users.

If we detect we are an unprivileged user, then insist that we
be in a userns with a id mapping.

If we are in a userns, then don't extract /dev when extracting
the rootfs.

If we are not root, then save the tarball to ~/.cache/lxc/cirros
instead of /var/cache/lxc/cirros.

If we are not roo, then include entries to auto-mount proc and sys,
as well as bind-mount devices.

Cc: Scott Moser <smoser@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>