Do some minimal work to better conform to the 802.3ad (LACP) standard.
In particular, don't set the synchronized bit for the peer unless it truly
appears to be synchronized to us. Also, don't set our own synchronized bit
unless we have actually seen a remote system.

Prior to this change, we were seeing some strange behavior, such as:

1. We send an advertisement with the Activity, Aggregation, and Default
flags, followed by an advertisement with the Activity, Aggregation,
Synchronization, and Default flags. However, we hadn't seen an
advertisement from another peer and were still advertising the default
(NULL) peer. A closer examination of the in-kernel data structures (using
kgdb) showed that the system had added the default (NULL) peer as a valid
aggregator for the segment.
2. We were receiving an advertisement from a peer that included the
default (NULL) peer instead of including our system information. However,
we responded with an advertisement that included the Synchronization flag
for both our system and the peer. (Since the peer's advertisement did not
include our system information, we shouldn't add the synchronization bit
for the peer.)

This patch corrects those two items.