disable broadcast if broadcast is set to net.IPv4zero

[WeidiDeng]

Another go at fixing 329.

According to comment, broadcast address auto-calculation is now a feature, and there is no intention to revert that. However, sometimes users do not want broadcast to be enabled.

Currently this can be done by setting Broadcast to net.IPv4zero. But that's not a public api of the netlink. And in my test, it will cause an error to be generated in the kernel log like attribute type 4 has an invalid length..

I think we can use this as a special value to disable broadcast since it already works. But now we explicitly set it to nil when interacting with netlink.

This is an alternative to introducing a new function or extending the Addr struct.

[WeidiDeng]

@aboch Do you have time to take a look at this change? What's your opinion?

[aboch]

Thank you @WeidiDeng.
Deviating from iproute2 behavior is not good. We have chosen semantic versioning for this package to be able to introduce API breaking changes.

Let's do the right change, let's make the behavior of the addr add/del methods in line with iproute2 behavior.
Then we can release a 2.0 version,
WDYT?

[WeidiDeng]

I think it's OK.