nsenter: fix setns() for rhel6 (glibc-2.12)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #12 from vbatts/vbatts-fix_setns_rhel6

nsenter: fix setns() for rhel6 (glibc-2.12)