user v0.1.0

This integrate the github.com/opencontainers/runc/libcontainer/user package
at commit opencontainers/runc@a3a0ec48c464fe6458d2ba067f6707c027e5384e.

This package was originally authored in a "utils" package in the Docker
repository, after which it moved to "libcontainer", which became part of
runc.

Some commits were not included in history, due to them being applied in
the Docker repository before moving into "libcontainer". Leaving links
to those for future reference:

• docker/docker@eb38750
• docker/docker@e41507b
• docker/docker@b07314e