add asserts as suggested.

I removed the "black magic" I added of relying upon alloc_unr_specific to return -1 when a user requested uplink0.
What was there before is more obvious and reads almost like english:

if (linkNum == 0 && isUplink)
        return (EINVAL);

That says exactly why we return EINVAL. Also I still get benefit of initializing unr(9) to start at 1 in the code. No longer have to grab a number and make sure its at least 1 for uplink.

I'm not sure how to clear the comments as they are often on wrong line numbers now. Sorry about that.

I have incorporated all the suggestions and uplink has a separate unr(9) now. Note the bhyve change is removed entirely as it is not necessary, you can still use peerhook=link with bhyve though for it to auto assign.

I have to clean up my patch and figure out how to update (I'm new to this). I will be removing bhyve from this so apologies to all the bhyve folks that got tagged on this.

Actually before I go move things to newhook can I ask do we have consensus that I should just have a separate unr(9) for uplink?
Also, the bhyve works without having it "figure out" it has an ng_bridge(4). You can pass "link" to it and it does the right thing. Given that should I pull bhyve change out so the patch is more focused on ng_bridge(4) which is my main concern?

In D44615#1017240, @glebius wrote:

Now more fundamental question, that you probably have answer for, cause you did it the way you did: why can't we alloc_unr() and possible return EEXIST in the (almost) synchronous newhook method? Why do we postpone that to later connect method?

In D44615#1017227, @afedorov wrote:

My main objection is that backward compatibility is broken. We can no longer have uplink[N] and link[N] at the same time. This may break existing software.

I had the same concern and point this out in the bug report. I am still not sure why uplink0 was never allowed, but kept that restriction. uplink is for when you attach a real network interface, an ng_ether(4) and although netgraph makes just about anything possible, I wonder if anybody does have more than one uplink on an ng_bridge(4)?