Initial version.
* Linux namespaces class abstractions with automatic resource control.
* Opening existing namespaces using PIDs.
* Opening parent user namespaces. (usually unaccessible from `/proc`)
* Switching to a namespace.
* Unsharing namespaces either from class method or function with boolean flags.
* Automatic file descriptor resource control using `with`.
* Getting and setting the max number of namespaces.
* Mount utilities using new file descriptor based API.
* Create bind mounts.