powerwake should make a best-effort attempt to guess broadcast

When I powerwake by hostname or IP, powerwake knows what network that is, and if we have a route for it, we should hit that subnet's broadcast, rather than 255.255.255.255, which is less reliable.

Example here:

(base)adconrad@lucifer:~$ ip ro sh
default via 68.145.116.1 dev eth0
10.0.0.0/27 dev br0 proto kernel scope link src 10.0.0.1
68.145.116.0/22 dev eth0 proto kernel scope link src 68.145.119.149

And a powerwake to 10.0.0.14 only works if I send it explicitly to 10.0.0.31. This should be easily guessable without me having to hand-hold.