lsns: show namespaces only kept alive by open file descriptors

lsns: show namespaces only kept alive by open file descriptors

Close #1884.

Quoted from the original issue comment submitted by @hesch:

  It can happen, that a namespace is only kept alive by an open file
  descriptor of a program as ilustrated by A.B:

    1. 'ip netns add foo' - add a namespace
    2. 'sleep 999 4< /run/netns/foo & sleep 2' - open the fd to the
       namespace in a background job
    3. 'ip netns delete foo' - delete the namespace (only deletes
       the /run/netns/foo)

  Now there exists a namespace with no process running in it and it has
  no bind mount so it does not show up in /proc/mounts, but it is still
  there and could be mounted back.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>