lxcfs, bindings: share cgroup info + add clone()

lxcfs, bindings: share cgroup info + add clone()

- So far lxcfs.c and bindings.{c,h} did not share cgroup information and we
  unnecessarily kept parsing and storing additional information. Let's share it
  instead.
- Add lxcfs_clone() function.
- Mount cgroups in a private mount namespace. We use CLONE_FILES so that file
  descriptors opened via lxcfs_clone() are not copied and hence are valid in
  child and parent.
- For each mounted hierarchy, open a file descriptor and store it in an mmap()ed
  array that is MAP_SHARED between parent and child.

Signed-off-by: Christian Brauner <cbrauner@suse.de>