Add helper nm_platform_link_get_ifi_flags() to access the
ifi-flags.

This replaces the internal API _link_get_flags() and makes it public.
However, the return value also allows to distinguish between errors
and valid flags.

Also, consider non-visible links. These are links that are in netlink,
but not visible in udev. The ifi-flags are inherrently netlink specific,
so it seems wrong to pretend that the link doesn't exist.