Add msg_enable and use netif_msg_link to enable/disable reporting
of link status changes since some Ethernet switches seem to generate
a lot of status changes under some circumstances and some people
want to avoid useless flooding in the logs.

Also add a counter for link status changes to statistics.
Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>