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)
Update the "cgroup-lite" dep in our deb package from "Suggests" to "Recommends" and add "cgroupfs-mount" as another alternative

I tested to verify that if neither package is available (for example, on Debian Wheezy), apt still continues installing properly.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
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
Merge pull request #5450 from tianon/update-deb-cgroup-dep

Update the "cgroup-lite" dep in our deb package from "Suggests" to "Recommends" and add "cgroupfs-mount" as another alternative
UTIME_OMIT is only available on Linux

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Add cpu throttling stats.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #5389 from tiborvass/5152-symlink-in-volume

Fixes #5152 : symlink in volume path
Another test to check for invalid stats.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #5455 from rjnagal/cgroup-stats

Add throttling stats for cpu cgroup
Merge pull request #5443 from ncdc/commit-api-doc-fix

Docs fix: correct /commit info
Merge pull request #5441 from ostezer/docs-fix-notes-warnings-bg

Docs/CSS: Fix Notes/Warnings blocks' BG colour to improve its state.
Merge pull request #5439 from ostezer/docs-mkdocs-theme-maintainer-update

Docs/theme/MAINTAINERS: Modify/Update list of MAINTAINERS
Merge pull request #5414 from jamtur01/privreg

Addressed regression of private repository documentation.
add redirects from index/ -> docker-io/ and for the docker-io_api too

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5438 from ostezer/docs-expand-viewport

Docs/CSS fix: Allow viewport expand to user's preference
remove the sphinx validation - we'll add a MarkDown one when we have it

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)