Add option parsing to nsenter, a privileged mode flag and enable specifying commands with arguments

1. Adds option parsing.
2. Adds a flag to nsenter as root with full capabilities. This allows admins to modify settings in the container namespaces.
3. As a side effect of using getopt option parsing, commands can be specified with arguments by using -- cmd arg1 arg2..

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)