Add dockerinit detection for using a separate static dockerinit

The logic here is as discussed with @shykes via IRC:
- if _DOCKER_INIT_PATH is set and is the path to an executable, just use that (especially for "go test")
- if the "docker" binary has a "dockerinit" executable in the same directory, use that
- if "/usr/libexec/docker/dockerinit" exists, use that (esp. for packagers)
- otherwise, fall back to using "docker" itself as dockerinit