Add --userland-proxy daemon flag

The `--userland-proxy` daemon flag makes it possible to rely on hairpin
NAT and additional iptables routes instead of userland proxy for port
publishing and inter-container communication.

Usage of the userland proxy remains the default as hairpin NAT is
unsupported by older kernels.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Upstream-commit: f42348e18f73d1d775d77ac75bc96466aae56d7c
Component: engine

components/engine/daemon/network/settings.go

@@ -15,4 +15,5 @@ type Settings struct {
 	Bridge                 string
 	PortMapping            map[string]map[string]string // Deprecated
 	Ports                  nat.PortMap
+	HairpinMode            bool
 }