Fix missing multicast cost field

cmd/yggdrasil/main.go

@@ -247,6 +247,7 @@ func main() {
 				Listen:   intf.Listen,
 				Port:     intf.Port,
 				Priority: uint8(intf.Priority),
+				Cost:     uint8(intf.Cost),
 				Password: intf.Password,
 			})
 		}

contrib/mobile/mobile.go

@@ -97,6 +97,7 @@ func (m *Yggdrasil) StartJSON(configjson []byte) error {
 				Listen:   intf.Listen,
 				Port:     intf.Port,
 				Priority: uint8(intf.Priority),
+				Cost:     uint8(intf.Cost),
 				Password: intf.Password,
 			})
 		}

src/config/config.go

@@ -62,6 +62,7 @@ type MulticastInterfaceConfig struct {
 	Listen   bool
 	Port     uint16
 	Priority uint64 // really uint8, but gobind won't export it
+	Cost     uint16 // really uint8, but gobind won't export it
 	Password string
 }