ipn/ipnlocal: add PROXY protocol support to Funnel/Serve

This adds the --proxy-protocol flag to 'tailscale serve' and
'tailscale funnel', which tells the Tailscale client to prepend a PROXY
protocol[1] header when making connections to the proxied-to backend.

I've verified that this works with our existing funnel servers without
additional work, since they pass along source address information via
PeerAPI already.

Updates #7747

[1]: https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

Change-Id: I647c24d319375c1b33e995555a541b7615d2d203
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

ipn/ipn_clone.go

@@ -218,10 +218,11 @@ func (src *TCPPortHandler) Clone() *TCPPortHandler {
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _TCPPortHandlerCloneNeedsRegeneration = TCPPortHandler(struct {
-	HTTPS        bool
-	HTTP         bool
-	TCPForward   string
-	TerminateTLS string
+	HTTPS         bool
+	HTTP          bool
+	TCPForward    string
+	TerminateTLS  string
+	ProxyProtocol int
 }{})
 
 // Clone makes a deep copy of HTTPHandler.