This adds support for a runtime configuration file that can do:
- `mkdir` to make a directory at runtime, eg in `/var` or `/tmp`, to avoid workarounds
- `interface` that can create network interfaces in a container or move them
- `bindNS` that can bind mount namespaces of an `onboot` container to a file so a service can be started in that namespace.
It merges the `service` and `onboot` tools (in `init`) to avoid duplication. This also saves some size for
eg LCOW which did not use the `onboot` code in `runc`.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
