The way the broadcast IP is currently fetched is by sending an ioctl()
request. The limitation of this is that the broadcast address may not
be set on the network interface (more specifically, it is usually not
set in docker containers). In those cases, it results in the broadcast
address becoming 0.0.0.0, which is not valid [1].
This fix uses the network IP and the netmask to compute the directed
broadcast address. This should be more reliable, as both of those are
always set on the network interface.
[1]: https://tools.ietf.org/html/rfc1122, section 3.2.1.3 (a)
Jonas Vautherin