Support startup without a tun device (#269)

This commit adds support for Nebula to be started without creating
a tun device. A node started in this mode still has a full "control
plane", but no effective "data plane". Its use is suited to a
lighthouse that has no need to partake in the mesh VPN.

Consequently, creation of the tun device is the only reason nebula
neesd to be started with elevated privileged, so this example
lighthouse can also be run as a non-root user.

main.go

@@ -101,11 +101,14 @@ func Main(config *Config, configTest bool, block bool, buildVersion string, logg
 	// tun config, listeners, anything modifying the computer should be below
 	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 
-	var tun *Tun
+	var tun Inside
 	if !configTest {
 		config.CatchHUP()
 
-		if tunFd != nil {
+		switch {
+		case config.GetBool("tun.disabled", false):
+			tun = newDisabledTun(tunCidr, l)
+		case tunFd != nil:
 			tun, err = newTunFromFd(
 				*tunFd,
 				tunCidr,
@@ -114,7 +117,7 @@ func Main(config *Config, configTest bool, block bool, buildVersion string, logg
 				unsafeRoutes,
 				config.GetInt("tun.tx_queue", 500),
 			)
-		} else {
+		default:
 			tun, err = newTun(
 				config.GetString("tun.dev", ""),
 				tunCidr,