It uses the newuidmap and newgidmap program to start a shell in
a mapped user namespace.  While newuidmap and newgidmap are
setuid-root, lxc-usernsexec is not.

If new{ug}idmap are not available, then this program is not
built or installed.  Otherwise, it will be used to support creating,
starting, destroying, etc containers by unprivileged users using
their authorized subuids and subgids.

Example:
	usernsexec -m u:0:100000:1 -- /bin/bash

will, if the user is authorized to use subuid 100000, start a
bash shell in a user namespace where 100000 on the host is
mapped to root in the namespace, and the shell is running as
(privileged) root.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>