cli: drop invalid validation for default-routes in nmcli
Currently, default-routes cannot be added like regular static-routes as ipv4.routes setting. Instead, one has to configure "ipv4.gateway" and "ipv4.never-default". That of course should be fixed, for example to configure a default-route in different routing tables. As it is, both nmcli's parse function and libnm's NMSettingIPConfig:verify() functions reject default-routes. But nmcli goes way beyond that, it also rejects all networks with "0.0.0.0"/"::" even if their prefix length is not zero. Such routes are not default-routes, and nmcli has no business rejecting them. The correct way for checking for a default-route is to check the prefix-length for zero. Drop the wrong validation in nmcli. Note, it may still not be the best idea to add catch-all routes like "0.0.0.0/1" and "126.96.36.199/1". It just defeats what counts as a default-route. NM has other means (like configuring the route-metric) to handle routing in face of multiple interfaces. But sure, whatever works for you.