Sometimes "ip netns delete" fails because it can not delete the file a
network namespace was mounted on.  If this only happened when a
network namespace was really in use this would be fine, but today it
is possible to pin all network namespaces by simply having a long
running process started with "ip netns exec".

Every mount is copied when a network namespace is created so it is
impossible to prevent the mounts from getting into other mount
namespaces.  Modify all mounts in the files and subdirectories of
/var/run/netns to be shared mount points so that unmount events can
propogate, making it unlikely that "ip netns delete" will fail because
a directory is mounted in another mount namespace.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>