team: allow userspace to take control over carrier

Some modes don't require any special carrier handling so
in these cases, the kernel can control the carrier as for
any other interface.  However, some other modes, e.g. lacp,
requires more than just that, so userspace needs to control
the carrier itself.

The daemon today is ready to control it, but the kernel
still can change it based on events.

This fix so that either kernel or userspace is controlling
the carrier.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>