The mesh's interface takes over the found tunnel's MTU

Carries MTU from the reported tunnel (mesh-host#28) through inventory,
the overlay graph's TakeOver, into the generated config's [Interface].
A tuned path keeps its MTU across the takeover instead of regressing to
1420 and hanging transfers no ping would reveal. Two emit tests; a
tunnel with no MTU writes no line.

internal/link/protocol.go

@@ -93,6 +93,7 @@ type Tunnel struct {
 	Unit      string       `json:"unit"`
 	Config    string       `json:"config"`
 	Port      int          `json:"port"`
+	MTU       int          `json:"mtu,omitempty"`
 	Address   string       `json:"address"`
 	Range     string       `json:"range"`
 	PublicKey string       `json:"public_key"`