Fix bug in preserve_ns

If /proc/self/ns does not exist, then preserve_ns was failing to
initialize the saved_ns[i] to -1.  This caused attach_ns() to try
and attach, and of course fail.

Initialize the saved ns values before returning an error.

The return values of preserve_ns and attach_ns were also being
ignored.  Honor them.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
One more rename of the system options

This should be the last change of name for those options. This change
basically move them all to lxc.bdev.<backend>.<option>.

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