lxc_user_nic changes

1. catch a case if allowed number of nics is 0 (as opposed to no entry),
which previously would have allowed the user to sneak in one entry.

2. return false, not true, if something went heywire with creating the
/sys/class/net/$nic path to open.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>