Update net-routing-setup to work for an exclusive-IP, DHCP, non-global zone
For a DHCP global zone, net-routing-setup (svc:/network/routing-setup:default) will use DHCP Router if defined as a signal to disable routeadm/default-ipv4-routing. net-routing-setup does nothing for an exclusive-IP, DHCP, non-global zone, but it could do easily the same.
Also, net-routing-setup only tries once to fetch Router from dhcpagent (dhcpinfo), which generally works for a global zone. For fast-booting non-global zones, there needs to be a few seconds allowance — so update to retry checks for a short interval. (The repeating query will be OK as well for a global zone).