expose link statistics as part of `getifaddrs`

[Luca-spopo]

Hello, first of all thank you for maintaining this amazing project.

nix::ifaddrs::InterfaceAddress is missing the field ifa_data, which is present in libc::ifaddrs

libc's ifa_data of type *mut c_void can point to a struct of type rtnl_link_stats when the address family is AF_PACKET. This struct contains useful statistics about the interface.

I would like to be able to get that info from the call to nix's getifaddrs.

My proposal is that we can expose a new field in InterfaceAddress:

pub link_stats: Option<LinkStats>,

LinkStats would be our representation of the fields in rtnl_link_stats which we can write by hand, or we can use the rtnl_link_stats struct provided by linux_raw_sys

I can make a PR for this code change if the proposal makes sense