setsockopt() handling of kern.ipc.maxsockbuf limit

[Cameron Sparr]

Sent an email yesterday already but I'm just bumping this one as I've
submitted a something through bugzilla to track this:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204438

I also added a little more details there comparing the behavior to how
Linux handles the same setsockopt function.