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, stage1: Refactor some code into functions

Will be used by enter command in further commit.
rkt, stage0, stage1: Add enter command

It should be used as follows:
rkt enter IMAGE [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 is basically a hack - we call systemd-nspawn without
boot parameter and we don't create any unit files. Because of that we
need to pass different directory parameter to systemd-nspawn - namely
stage2 directory.