Fix https://github.com/jpetazzo/nsenter/issues/21

Prefix the shell command with `sudo ` if the user is not `root`.
Assumes that `sudo` exists.
Sorry, my answer was very terse :-)

By default, if you `$FOO` when `FOO` is not defined, it just expands to an empty string.

But you can also do `set -u`; then if you reference an undefined variable, it causes an error.