UDS listen sockets restrict VCL to >= 4.1

I had hoped for a more explanatory error message, but that is too
involved at this point.

bin/varnishd/mgt/mgt_acceptor.c

@@ -268,6 +268,9 @@ MAC_Arg(const char *spec)
 		ARGV_ERR("Unix domain socket addresses must be absolute paths "
 			 "in -a (%s)\n", la->endpoint);
 
+	if (*la->endpoint == '/' && heritage.min_vcl < 41)
+		heritage.min_vcl = 41;
+
 	for (int i = 2; av[i] != NULL; i++) {
 		char *eq, *val;
 		int len;