sd-rtnl: add sd_rtnl_message_route_get_family
sd-rtnl: message - allow checking for attributes without reading out their contents
sd-rtnl: add a bit more debugging in case a message is dropped
sd-rtnl: add typesystem for RTM_*NEIGH
sd-rtnl: add sd_rtnl_message_{new_neigh,neigh_get_{family,ifindex}}
shared: udev-util - add hwdb cleanup macro
sd-rtnl: route - allow GETROUTE with AF_UNSPEC
networkctl: print the Gateway in the status output

This is the IP address of the default route on the link, if present. A
description is printed when available (the manufacturer of the gateway NIC based
on its MAC address).

In the future we should prefer LLDP information over MAC info.