add interface_id in ScopedIpV4

[keepsimple1]

Help fixing issue #437

Problem

Currently for IPv4 there is no way to find out on which interface the service address(es) are found, while IPv6 doesn't have this problem.

Solution

Added interface_id() to ScopedIpV4 to match ScopedIpV6 that returns the interface on which a service address is found.

[hgomersall]

It looks like this would certainly allow a workaround. I think probably by iterating over the interfaces from if_addrs and comparing each with the returned InterfaceId. That said, is there a reason the address can't be added to the interface ID directly from if_addrs to avoid the extra layer?

[keepsimple1]

That said, is there a reason the address can't be added to the interface ID directly from if_addrs to avoid the extra layer?

I didn't want to do that as it feels like against the design purpose of struct InterfaceId, but I think we could add a convenient method to do the legwork for the users. Please see the updated diff for get_addrs() method of InterfaceId. Would that work for you?