networkd: Static neighbor support

When using networkd we currently have no way of ensuring that static
neighbor entries are set when our link comes up. This change adds a new
section to the network definition that allows multiple static neighbors
to be set on a link.

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

@@ -165,6 +165,9 @@ EmitDNS=
 EmitDomains=
 Managed=
 OtherInformation=
+[Neighbor]
+Address=
+MacAddress=
 [IPv6AddressLabel]
 Label=
 Prefix=