Merge pull request #137 from mrunalp/dev/moveuplockthread

Lock the thread first thing in init.
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)