networkd: add RouteDenyList

Allow configuration for IPv6 discovered routes to be ignored instead of
adding them as a route. This can be used to block unwanted routes, for
example, you may wish to not receive some set of routes on an interface
if they are causing issues.

test/fuzz/fuzz-network-parser/directives.network

@@ -312,6 +312,7 @@ UseAutonomousPrefix=
 UseOnLinkPrefix=
 DenyList=
 BlackList=
+RouteDenyList=
 [DHCPServer]
 EmitNTP=
 PoolSize=