Added ability to create dynamic docker and static dockerinit through make scripts directly

This is the result of a long and hairy discussion with @shykes on IRC regarding separate dockerinit.

The logic for docker to officially find `dockerinit` is as follows:
- if `_DOCKER_INIT_PATH` is set and is the path to an executable, we use that (especially for "go test" via `./hake/make.sh dynbinary dyntest`)
- if the docker binary has a dockerinit executable in the same directory, we use that
- if `/usr/libexec/docker/dockerinit` exists, we use that (esp. for packagers)
- otherwise, we fall back to using docker itself as dockerinit, as we always have

I've also included a documentation update to `hack/PACKAGERS.md` regarding this last-resort method of compiling docker dynamically with a static dockerinit.