ns: Add c/r for /proc/$pid/ns/$ids references

Based on work done by Cyrill Corcunov (many thanks for that).

In this commit we implement c/r for files which have opened
/proc/$pid/ns/$ids entries.

The idea is rather simple one

Checkpoint
==========

- Check if the file name is the one of known to be ns ref
- If match then write protobuf entry

Restore
=======

- Read all ns entries from the image
- When criu tries to open one we lookup over process
  tree to figure out which PID should be used in path
  and then just open it

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