cmd/tailscale,ipn: add Unix socket support for serve

Based on PR #16700 by @lox, adapted to current codebase.

Adds support for proxying HTTP requests to Unix domain sockets via
tailscale serve unix:/path/to/socket, enabling exposure of services
like Docker, containerd, PHP-FPM over Tailscale without TCP bridging.

The implementation includes reasonable protections against exposure of
tailscaled's own socket.

Adaptations from original PR:
- Use net.Dialer.DialContext instead of net.Dial for context propagation
- Use http.Transport with Protocols API (current h2c approach, not http2.Transport)
- Resolve conflicts with hasScheme variable in ExpandProxyTargetValue

Updates #9771

Signed-off-by: Peter A. <ink.splatters@pm.me>
Co-authored-by: Lachlan Donald <lachlan@ljd.cc>

cmd/tailscaled/tailscaled.go

@@ -401,6 +401,7 @@ func run() (err error) {
 	// Install an event bus as early as possible, so that it's
 	// available universally when setting up everything else.
 	sys := tsd.NewSystem()
+	sys.SocketPath = args.socketpath
 
 	// Parse config, if specified, to fail early if it's invalid.
 	var conf *conffile.Config