Move "pkg/user" into libcontainer...

... and add support for `GetUserGroupSupplementary` to return "Home" too

Fixes #94

See also dotcloud/docker#2968 (for the `HOME` stuff)
Just for a little background here, we already call `LoadContainerEnvironment` earlier in this function which does `os.Clearenv` and then `os.Setenv` for each key-pair in `container.Env`, and then `SetupUser` also sets `HOME` if it wasn't already set in `container.Env`.