[lacp] Set "aggregatable" flag in response LACPDU

[lacp] Set "aggregatable" flag in response LACPDU

Some switches do not allow an individual link (as defined in IEEE Std
802.3ad-2000 section 43.3.5) to work alone in a link aggregation group
as described in section 43.3.6.  This is verified on Dell's
PowerConnect M6220, based on the Broadcom Strata XGS-IV chipset.

Set the LACP_STATE_AGGREGATABLE flag in the actor.state field to
announce link aggregation in the response LACPDU, which will have the
switch enable the link aggregation group and allow frames to pass.

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