[BRIDGE]: linkstate handling

[BRIDGE]: linkstate handling

This makes bridge port status reflect both the state of the interface
from software (up/down) and the carrier.  It makes STP handle link failure
(cable breakage, etc).  The original concept comes from a
Mark Ruijter <bridge@siennax.com> who implemented it differently.
My way is simpler and requires no polling.

Obviously, this link state detection will only work if the network card
handles the events properly.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@redhat.com>