Refactor and expose private functions within `libcontainer/user`.

This patchset refactors much of the code in `user/` (such as using structs to return user data and general readability improvements), exposes several previously private functions (`parsePasswdFile`, `parseGroupFile`) to allow for callers to pass `io.Reader`s as a data source (massively improving the usability of the package) and adds several exhaustive unit tests for RunUser parsing.

Also, the gargantuan function signature `user.GetUserGroupSupplementaryHome` was changed to the more readable and understandable `user.GetExecUser`.

This patch was created to assist the implementation of docker/docker#7537, in order to ensure the RunUser configuration parsing can be handled consistently both internally to libcontainer, as well as external to libcontainer.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
/ping @tianon @crosbymichael Any thoughts about my argument?