Move nsenter code into separate package

Signed-off-by: Michael Crosby <michael@docker.com>
Import nsenter code so that it is compiled into nsinit

Signed-off-by: Michael Crosby <michael@docker.com>
Nsenter checks for argv 0 before running

Signed-off-by: Michael Crosby <michael@docker.com>
Modify nsinit to register actions for argv 0

Signed-off-by: Michael Crosby <michael@docker.com>