core: rename NM_PLATFORM_ROUTE_METRIC_DEFAULT to NM_PLATFORM_ROUTE_METRIC_DEFAULT_IP6

For IPv4, iproute for example defaults to a metric of 0.
Hence, the name NM_PLATFORM_ROUTE_METRIC_DEFAULT was misleading.

Also add a NM_PLATFORM_ROUTE_METRIC_DEFAULT_IP4 define for completeness.

https://bugzilla.gnome.org/show_bug.cgi?id=740780
iface-helper: disambiguate route priority for IPv4 and IPv6

https://bugzilla.gnome.org/show_bug.cgi?id=740780
ethernet: Link may have vanished
Merge branch 'lr-veth-rm', remote-tracking branches 'fd/lr/ipv6-tokens', 'fd/lr/veth-managed', 'fd/lr/no-udev' and 'fd/lr/vpn-fixes' into integration