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>