Instead of asking systemd-nspawn to write its "pid" file, rkt can write
the parent pid "ppid". Then, update getPID() to get the pid file from
/proc/$ppid/task/$ppid/children. In this way, we don't need to patch
systemd-nspawn.
getPID() is able to read both the "pid" file and the "ppid" file, in
order to work with alternative stage1s which choose to implement one or
the other method.
coreos/rkt#910
Add a script to copy busybox into stage1 for debugging purposes. It
takes a container UUID and outputs the nsenter command needed to enter
stage1's namespaces.
When using overlay filesystem, a new mount namespace will be created for
stage1. This script enters this namespace to copy busybox.
