This gives the receiver of the PROXY header (usually the ssl-onloader)
the opportunity to set the SNI (HostName field) from the TLV value, for
the TLS handshake with the remote backend.

From
https://github.com/nigoroll/varnish-cache/commit/e0eb7d0a9c65cdc3c58978656b4c71f4ab8aabca
edited by @nigoroll to split out the proxy header functionality.

Add vmod_debug access to the proxy header formatting and test it