Fix container.json sample to be loadable by nsinit.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Fix typos in nsinit logs.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Add enabled option to namespaces and capabilities spec in
container.json. Although we don't yet check for enabled everywhere.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Improved README formatting.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Updated sample config and README to match the default template for
native execdriver.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Updated sample config to be usable. We should change the namespace
config to not need "value" later.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Remove "root" and "" special cases in libcontainer

These are unnecessary since the user package handles these cases properly already (as evidenced by the LXC backend not having these special cases).

I also updated the errors returned to match the other libcontainer error messages in this same file.

Also, switching from Setresuid to Setuid directly isn't a problem, because the "setuid" system call will automatically do that if our own effective UID is root currently: (from `man 2 setuid`)

    setuid() sets the effective user ID of the calling process.  If the
    effective UID of the caller is root, the real UID and saved set-user-
    ID are also set.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Adding a unit test for pkg/cgroup/fs/memory.go

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge branch 'master' into libcontainer-fixes

Conflicts:
	pkg/libcontainer/README.md
	pkg/libcontainer/container.json

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #5392 from rjnagal/libcontainer-fixes

Minor libcontainer fixes
Merge pull request #5449 from tianon/remove-libcontainer-root-special-case

Remove "root" and "" special cases in libcontainer
Merge pull request #5451 from vmarmol/add-memory-stats

Adding a unit test for stats in pkg/cgroup/fs/memory.go