A found tunnel carries its MTU to the mesh

The host parses MTU from the found [Interface] and reports it, so the
mesh's interface can come up with the same MTU when it takes the tunnel
over. A path tuned to 1380 regresses to the 1420 default otherwise —
invisible to ping, fatal to TLS handshakes and transfers over that path
(novox/hq: the mesh had no MTU concept). Zero when the config named
none, and the mesh writes no MTU line then.

internal/link/enrol.go

@@ -68,6 +68,7 @@ type Tunnel struct {
 	Port      int          `json:"port"`
 	Address   string       `json:"address"`
 	Range     string       `json:"range"`
+	MTU       int          `json:"mtu,omitempty"`
 	PublicKey string       `json:"public_key"`
 	Peers     []TunnelPeer `json:"peers,omitempty"`
 }