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

Remove "root" and "" special cases in libcontainer