Mark the link as blocked if the LACP partner is not reporting itself
as being in sync, collecting, and distributing.

This matches the behaviour for STP: we mark the link as blocked if we
detect that the switch is actively blocking traffic, in order to
extend the DHCP discovery period and so prevent boot failures on
switches that take an excessively long time to enable ports.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Relicense files for which I am the sole author (as identified by
util/relicense.pl).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Some switch configurations will refuse to enable our port unless we
can speak LACP to inform the switch that we are alive.  Add a very
simple passive LACP implementation that is sufficient to convince at
least Linux's bonding driver (when tested using qemu attached to a tap
device enslaved to a bond device configured as "mode=802.3ad").

Signed-off-by: Michael Brown <mcb30@ipxe.org>