Add support for haproxy proxy protocol

Currently all ssh connections are from from localhost, and the original client address is not available to sshd.

The HAProxy proxy protocol allows the client address to be preserved for non-http connections like ssh.

tunnel origin request configuration should have an option to send a proxy header at the start of the connection. This would allow client address to be restored via [mmproxy](https://github.com/cloudflare/mmproxy).

Note this is already supported by spectrum (https://developers.cloudflare.com/spectrum/proxy-protocol).