System manager interactions for FRR

[donaldsharp]

See 2nd commit for a description of what is being done, as that it holds the design I worked towards.

Start the ability for FRR to interact with a system manager. Create test commands, ensure that they work.

[mjstapp]

I guess my initial reaction to the document is that I don't understand what this is actually ... going to do?

Is zebra going to notice that "warm boot" is occurring? If so, what
will zebra do differently? Do any other daemons need to know about
that event, and if so, what will it/they do? Does zebra need to
distribute an event via zapi? Does it need to track who has received
and acknowledged the event? I'd rather see that proposal as a
standalone design to evaluate.

Similarly, what does "port reset" mean, and what will change in zebra
if ... it is happening? Again, I'd rather see that proposal, and
understand what that would mean for zebra, independently.

I think I understand the "GR" event you've described, but that could
just be a hook function, right? And that makes me wonder about all the
rest: I don't quite see why we'd need a new pthread and assorted
machinery just to set a boolean (for "warm reboot"), or to have zebra
call a "GR done" hook function.

I understand the machinery that's proposed, but I think it's premature
to choose the machinery to accomplish something when ... what is to be
done isn't clear (to me).