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/25-neighbor-section.network

@@ -0,0 +1,6 @@
+[Match]
+Name=dummy98
+
+[Neighbor]
+Address=2004:da8:1:0::
+MACAddress=00:00:5e:00:02:00

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

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