Adds mask_proxy_protocol config option (0 = off, 1 = v1 text, 2 = v2 binary)
that sends a PROXY protocol header when connecting to mask_host. This lets
the backend see the real client IP address.
Particularly useful when the masking site (nginx/HAProxy) runs on the same
host as telemt and listens on a local port — without this, the backend loses
the original client IP entirely.
PROXY protocol header is also sent during TLS emulation fetches so that
backends with proxy_protocol required don't reject the connection.
ivulit