wgengine/magicsock: introduce virtualNetworkID type (#16021)

This type improves code clarity and reduces the chance of heap alloc as
we pass it as a non-pointer. VNI being a 3-byte value enables us to
track set vs unset via the reserved/unused byte.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -1112,7 +1112,7 @@ func (de *endpoint) sendDiscoPing(ep netip.AddrPort, discoKey key.DiscoPublic, t
 	size = min(size, MaxDiscoPingSize)
 	padding := max(size-discoPingSize, 0)
 
-	sent, _ := de.c.sendDiscoMessage(ep, nil, de.publicKey, discoKey, &disco.Ping{
+	sent, _ := de.c.sendDiscoMessage(ep, virtualNetworkID{}, de.publicKey, discoKey, &disco.Ping{
 		TxID:    [12]byte(txid),
 		NodeKey: de.c.publicKeyAtomic.Load(),
 		Padding: padding,