* Support building with `unix-2.8`.
* Use raw POSIX io to write to proc files (Baojun Wang).
* Support denying setgroups to processes in user namespaces, needed to support unprivileged creation of user namespaces (Baojun Wang).
* Provide Read instances for UserMapping & GroupMapping.
* Improve documentation.
* Export data constructors for UserMapping & GroupMapping.