Move SysInit to a submodule

This allows docker-init to not import the main docker
module, which means it won't e.g. pick up any sqlite
dependencies.
Add a separate docker-init binary

This may be used for the .dockerinit case if the main binary is not
statically linked.

Conflicts:
	docker-init/docker-init.go
Rename all cases of "docker-init" to "dockerinit" for consistency
Merge branch 'separate-dockerinit' of https://github.com/tianon/docker into tianon-separate-dockerinit
Merge branch 'tianon-separate-dockerinit' into dm

Conflicts:
	runtime_test.go