Docker 'runin' demands passing flags before 'nsenter' cli option.
Docker does not require RunIn API. Hence that API has been removed.
nsinit CLI has been modified to work around the nsenter changes.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Check for "nsenter" in args before parsing flags. Addressed comments.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Remove debug message.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #141 from vishh/runin_support

Change nsenter to support docker 'runin'