lxcapi_destroy: run in a namespace if we are unprivileged

This is necessary to have the rights to remove files owned by our subuids.

Also update lxc_rmdir_onedev to return 0 on success, -1 on failure.
Callers were not consistent in using it correctly, and this is more
in keeping with the rest of our code.

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