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>

tsnet/depaware.txt

@@ -43,6 +43,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
    L 💣 github.com/mdlayher/netlink/nlenc                            from github.com/jsimonetti/rtnetlink+
   LA 💣 github.com/mdlayher/socket                                   from github.com/mdlayher/netlink+
  LDW 💣 github.com/mitchellh/go-ps                                   from tailscale.com/safesocket
+        github.com/pires/go-proxyproto                               from tailscale.com/ipn/ipnlocal
   DI    github.com/prometheus-community/pro-bing                     from tailscale.com/wgengine/netstack
    L 💣 github.com/safchain/ethtool                                  from tailscale.com/net/netkernelconf
    W 💣 github.com/tailscale/certstore                               from tailscale.com/control/controlclient