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/inventory/tunnel.go

@@ -33,6 +33,9 @@ type Tunnel struct {
 	// Port is the port the found interface listened on — one the hosting provider already lets
 	// through, which is why it is worth taking.
 	Port int `json:"port"`
+	// MTU is the found interface's, when it set one; the mesh's interface takes it over so a
+	// tuned path does not silently regress to the default (novox/hq: a taken tunnel carries its MTU).
+	MTU int `json:"mtu,omitempty"`
 	// Address is the interface's own address with its prefix length, 192.0.2.1/24; Range is the
 	// network that prefix names, 192.0.2.0/24.
 	Address string `json:"address"`