vcl: Make resp.proto read-only

One of the VCL 4.1 changes was to make the protocol read-only, which
we've successfully done for all variables except resp.proto that
kept the exact same definition for both 4.0 and 4.1 versions, despite
having distinct definitions.

bin/varnishtest/tests/v00001.vtc

 varnishtest "VCL/VRT: url/request/proto/reason/status"
 
+varnish v1 -errvcl "Variable is read only" {
+	sub vcl_deliver { set req.proto = "HTTP/1.2"; }
+}
+
+varnish v1 -errvcl "Variable is read only" {
+	sub vcl_deliver { set resp.proto = "HTTP/1.2"; }
+}
+
+varnish v1 -errvcl "Variable is read only" {
+	sub vcl_backend_response { set bereq.proto = "HTTP/1.2"; }
+}
+
+varnish v1 -errvcl "Variable is read only" {
+	sub vcl_backend_response { set beresp.proto = "HTTP/1.2"; }
+}
 
 server s1 {
 	rxreq

doc/sphinx/reference/vcl_var.rst

@@ -1191,8 +1191,6 @@ resp.proto	``VCL >= 4.1``
 
 	Readable from: vcl_deliver, vcl_synth
 
-	Writable from: vcl_deliver, vcl_synth
-
 	The HTTP protocol version to use for the response.
 
 resp.status