Adapt to latest spec
stage0: Split out a function execing init1

It will be used by enter command later.
stage1: Add command handling to init1

This moves most of the code that was in main to run function and now
main handles parameter parsing.

init1 should be run like as follows:
init1 CMD [debug] [-- [optional command specific parameters]]

run command has no optional parameters for now.
rkt, stage0, stage1: Add enter command

It should be used as follows:
rkt enter [optional binary to run [optional parameters to binary]]

If no optional binary is provided then root shell inside container
will be executed.

Under the hood it basically does similar things like run, but runs
systemd-nspawn without boot parameter and creates no additional unit
files.