Restore most cases of am_guest_unpriv

Restore most cases of am_guest_unpriv

The only cases where we really need to be privileged with respect
to the host is when we are trying to mknod, and in some cases
to do with a physical network device.  This patch leaves the
detection of the network device cases as a TODO.

This should fix the currently broken case of starting a privileged
container with at least one veth nic, nested inside an unprivileged
container.

Cc: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: Serge Hallyn <shallyn@cisco.com>