Enable Windows CI (Appveyor)

- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility

pkg/ip/route_linux.go

@@ -39,3 +39,9 @@ func AddHostRoute(ipn *net.IPNet, gw net.IP, dev netlink.Link) error {
 		Gw:        gw,
 	})
 }
+
+// AddDefaultRoute sets the default route on the given gateway.
+func AddDefaultRoute(gw net.IP, dev netlink.Link) error {
+	_, defNet, _ := net.ParseCIDR("0.0.0.0/0")
+	return AddRoute(defNet, gw, dev)
+}