restore: correctly restore cgroup mounts inside a container

Before the nsroot= mount option, we were just getting lucky because the
cgroup superblocks "matched" when inspecting them from userspace, so we
were actually getting a bind mount from the host when migrating from within
cgroup namespaces.

Instead, let's actually do a new (i.e. not a bind mount) for cgroup
namespaces. For this, we need two things:

1. to prepare the cgroup namespace (and thus the cgroups) before the mount
   ns, so when the mount() occurrs it is relative to the right cgroup path.

2. not reject cgroup filesystems with no root. A cgroup ns mount looks
   like:

	 223 222 0:22 /lxc/unpriv /sys/fs/cgroup/systemd rw,nosuid,nodev,noexec,relatime - cgroup cgroup rw,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd,nsroot=/lxc/unpriv

   i.e. it has /lxc/unpriv as its root, and thus doesn't look rooted to CRIU.
   We use the fstype->parse hook to rewrite this root to /, since it
   is handled by the cgroup ns infrastructure.

v2: add new fstype->munge hook, allowing fstypes to munge their parsed
    mountinfo entries if they want to. this allows us to get rid of the
    ugly hacks with FSTYPE__CGROUP everywhere in teh patch.
v3: s/fstype->munge/fstype->parse for FSTYPE__CGROUP

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>