netdev: New function netdev_ref().

I suspect that this makes it easier to make sure that a netdev stays open
as long as needed in some cases where a module needs access to a netdev
opened by some higher-level module.

CC: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
cfm: Implement "demand mode".

The new CFM "demand mode" (named after BFD's demand mode) uses
data traffic to indicate interface liveness.  It's helpful on
heavily congested networks where CCMs may be dropped.

Signed-off-by: Ethan Jackson <ethan@nicira.com>