Avoid hard coding network address for bridge

[swills]

Add functions to examine local subnets and avoid using a network that is
already in use locally. More may be needed, but this avoids a local issue for
now. Note this will only work on 11.0 and later since it uses the libxo support
in netstat.

[jesa7955]

Thank you for your great work! This commit looks good, but I think not all bhyve users have updated their FreeBSD to 11.0. Shouldn't we add a version check before searching an available subnet?

[swills]

Yeah, probably not everyone has upgraded, so we should support 10.x too. But instead of adding a version check and falling back to the hard coded value for 10.x, we can just add a way to get the netstat output that works on 10.x to _find_local_subnets. And that method would probably work on 11.x as well. I just didn't want to write the code to parse netstat output right now. Maybe I'll get motivated and do it tomorrow. Or maybe someone else will get there first. :)