handle unprivileged user calls more gracefully (v3)

handle unprivileged user calls more gracefully (v3)

Return an error if the function is not supposed to be called by an unprivileged user.
Otherwise those calls fail in the middle of their execution with different reasons.

changes since v2:
- am_unpriv is now a simple geteuid check,
- API functions are now providing error messages,
- lxc-info, lxc-attach are now checking geteuidi,
- lxc-ls is now calling get_ips only if the container is running

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>