Fix unprivileged containers started by root

Fix unprivileged containers started by root

This change makes it possible to create unprivileged containers as root.
They will be stored in the usual system wide location, use the usual
system wide cache but will be running using a uid/gid map.

This also updates lxc_usernsexec to use the same function as the rest of
LXC, centralizing all the userns switch in a single function.

That function now detects the presence of newuidmap and newgidmap on the
system, if they are present, they will be used for containers created as
either user or root. If they're not and the user isn't root, an error is
shown. If they're not and the user is root, LXC will directly set the
uid_map and gid_map values.

All that should allow for a consistent experience as well as supporting
distributions that don't yet ship newuidmap/newgidmap.

To make things simpler in the future, an helper function "on_path" is
also introduced and used to detect the presence of newuidmap and
newgidmap.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>