sni-router: collapse haproxy bind to comma-separated form

Switch to one-line `bind :80,[::]:80` and `bind :443,[::]:443` per
review feedback in #522.  The v6only flag was self-documentation, not
load-bearing: with SO_REUSEADDR (HAProxy's default) and bindv6only=0
the kernel routes v4 packets to the more-specific AF_INET socket
regardless.  Comment trimmed to match — the v6only paragraph is gone
because v6only itself is gone.

The shorter form also scales more cleanly when adding ports later,
e.g. `bind :8080,[::]:8080` on a new line.

contrib/sni-router/haproxy.cfg

 # --- HTTP :80 — ACME challenges + redirect -----------------------------------
 
 frontend http
-    # Explicit v4 + v6 binds so IPv6 clients are accepted regardless of the
-    # host's net.ipv6.bindv6only sysctl.  `v6only` on the v6 bind prevents it
-    # from also accepting v4-mapped connections, which would otherwise
-    # conflict with the explicit v4 bind on the same port.
-    bind 0.0.0.0:80
-    bind [::]:80 v6only
+    # Explicit v4 + v6 binds so IPv6 clients are accepted regardless of
+    # the host's net.ipv6.bindv6only sysctl.
+    bind :80,[::]:80
     mode http
 
     # Let Caddy answer ACME HTTP-01 challenges for Let's Encrypt.
 # --- TLS :443 — SNI-based routing -------------------------------------------
 
 frontend tls
-    bind 0.0.0.0:443
-    bind [::]:443 v6only
+    bind :443,[::]:443
     tcp-request inspect-delay 5s
     tcp-request content accept if { req_ssl_hello_type 1 }