Update Linux interface getting to make fewer syscalls

Each time we call if_nametoindex, if_indextoname or somesuch, the libc
needs to open a socket, make an ioctl, and close the socket. Since we've
got most of the information we need anyway in the data from
getifaddrs(3), let's just use it

Change-Id: I572c212a27c4b9ffe57980b36f75bb04e6d5cf29
Reviewed-by: Jonas Gastal <gastal@intel.com>
Reviewed-by: Richard J. Moore <rich@kde.org>