cmd/tailscale/cli: make advertise-{routes,exit-node} available unconditionally

It was only Linux and BSDs before, but now with netstack mode, it also works on
Windows and darwin. It's not worth limiting it to certain platforms.

Tailscaled itself can complain/fail if it doesn't like the settings
for the mode/OS it's operating under.

Updates #707

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -49,10 +49,8 @@ specify any flags, options are reset to their default.
 		upf.StringVar(&upArgs.advertiseTags, "advertise-tags", "", "ACL tags to request (comma-separated, e.g. \"tag:eng,tag:montreal,tag:ssh\")")
 		upf.StringVar(&upArgs.authKey, "authkey", "", "node authorization key")
 		upf.StringVar(&upArgs.hostname, "hostname", "", "hostname to use instead of the one provided by the OS")
-		if runtime.GOOS == "linux" || isBSD(runtime.GOOS) {
-			upf.StringVar(&upArgs.advertiseRoutes, "advertise-routes", "", "routes to advertise to other nodes (comma-separated, e.g. \"10.0.0.0/8,192.168.0.0/24\")")
-			upf.BoolVar(&upArgs.advertiseDefaultRoute, "advertise-exit-node", false, "offer to be an exit node for internet traffic for the tailnet")
-		}
+		upf.StringVar(&upArgs.advertiseRoutes, "advertise-routes", "", "routes to advertise to other nodes (comma-separated, e.g. \"10.0.0.0/8,192.168.0.0/24\")")
+		upf.BoolVar(&upArgs.advertiseDefaultRoute, "advertise-exit-node", false, "offer to be an exit node for internet traffic for the tailnet")
 		if runtime.GOOS == "linux" {
 			upf.BoolVar(&upArgs.snat, "snat-subnet-routes", true, "source NAT traffic to local routes advertised with --advertise-routes")
 			upf.StringVar(&upArgs.netfilterMode, "netfilter-mode", defaultNetfilterMode(), "netfilter mode (one of on, nodivert, off)")
@@ -86,10 +84,6 @@ var upArgs struct {
 	hostname              string
 }
 
-func isBSD(s string) bool {
-	return s == "dragonfly" || s == "freebsd" || s == "netbsd" || s == "openbsd"
-}
-
 func warnf(format string, args ...interface{}) {
 	fmt.Printf("Warning: "+format+"\n", args...)
 }