Make libblkio better as a Meson subproject
Any Meson project (libblkio included) could be pulled in as a subproject as it is. However, it usually takes a few tweaks to make project a good subproject: disable parts you don't need when pulling a dependency, or exposing some build artifacts for reuse in a parent project.
This PR adds some of these tweaks to libblkio: it makes docs and tests optional and exposes examples via an intentionally loose contract.