client/web,cmd/tailscale: add prefix flag for web command

We already had a path on the web client server struct, but hadn't
plumbed it through to the CLI. Add that now and use it for Synology and
QNAP instead of hard-coding the path. (Adding flag for QNAP is
tailscale/tailscale-qpkg#112) This will allow supporting other
environments (like unraid) without additional changes to the client/web
package.

Also fix a small bug in unraid handling to only include the csrf token
on POST requests.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/cli/web.go

@@ -39,6 +39,7 @@ Tailscale, as opposed to a CLI or a native app.
 		webf.StringVar(&webArgs.listen, "listen", "localhost:8088", "listen address; use port 0 for automatic")
 		webf.BoolVar(&webArgs.cgi, "cgi", false, "run as CGI script")
 		webf.BoolVar(&webArgs.dev, "dev", false, "run web client in developer mode [this flag is in development, use is unsupported]")
+		webf.StringVar(&webArgs.prefix, "prefix", "", "URL prefix added to requests (for cgi or reverse proxies)")
 		return webf
 	})(),
 	Exec: runWeb,
@@ -48,6 +49,7 @@ var webArgs struct {
 	listen string
 	cgi    bool
 	dev    bool
+	prefix string
 }
 
 func tlsConfigFromEnvironment() *tls.Config {
@@ -81,6 +83,7 @@ func runWeb(ctx context.Context, args []string) error {
 	webServer, cleanup := web.NewServer(ctx, web.ServerOpts{
 		DevMode:     webArgs.dev,
 		CGIMode:     webArgs.cgi,
+		PathPrefix:  webArgs.prefix,
 		LocalClient: &localClient,
 	})
 	defer cleanup()