Better handle preserve_ns behavior

Better handle preserve_ns behavior

Commit b6b2b194a8 preserves the container's namespaces for
possible later use in stop hook.  But some kernels don't have
/proc/pid/ns/ns for all the namespaces we may be interested in.
So warn but continue if this is the case.

Implement stgraber's suggested semantics.

 - User requests some namespaces be preserved:
    - If /proc/self/ns is missing => fail (saying kernel misses setns)
    - If /proc/self/ns/<namespace> entry is missing => fail (saying kernel misses setns for <namespace>)
 - User doesn't request some namespaces be preserved:
    - If /proc/self/ns is missing => log an INFO message (kernel misses setns) and continue
    - If /proc/self/ns/<namespace> entry is missing => log an INFO message (kernel misses setns for <namespace>) and continue

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>