libct: Introduce option for mounting cgroups inside CT

Cgroups created for container will be bind mounted as
root cgroup into container's FS.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Fix re-mounting CT's own /proc

Main change is -- we can have proc in either new
mount namespace or withint chroot.

Other than this, report error if mounting new proc
is impossible.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
uts: Don't forget to free host/domain-names

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
ct: Helper for setting uname values

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
cg: Implement bind-mounting cgroups subs into CT

Now only works for new mount namespaces with root. This is
possible for both simple chroot and for shared root in new
mount namespace, but requires more coding.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>