Transition usage of `libcontainer/user` to use new interface

This patchset transitions the use of the old `libcontainer/user` interface
to the new and improved `libcontainer/user` interface.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

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This patchset should be timed to be merged *after* docker/libcontainer#158, as this patchset essentially updates Docker's usage of the `libcontainer/user` interface to the interface implemented in docker/libcontainer#158.

/cc @vieux @tianon @crosbymichael