[WIP] Linux: set broadcast flag by default for ipvlan interfaces.

[ido]

The patch uses ethtool's SIOCETHTOOL ioctl to obtain the driver name of the interface, and turns on the broadcast flag by default when it detects an ipvlan interface. This does not yet differentiate between L2, L3, or L3S mode.

@rsmarples we can use this ethtool method to detect most other device types, like bridge and tuntap, without reading /sys. I might refactor those after this is cleaned up.

TODO:

• [ ] Modify DHCP IAID so that ipvlan interfaces do not generate an IAID conflict: ipvlan0: IAID conflicts with one assigned to eth0
• [x] Clean up patch before merging.

Fixes #32

[deliciouslytyped]

Please remember to change the documentation as well!

[ido]

I squashed the commits into one patch. So far, this sets broadcast. I might have a solution for setting unique IAIDs soon.