Use "none" to disable authentication

Previously you needed an empty argument (''), which didn't play very
well along sysv init scripts and was not documented.
This should address both problems.

Fixes #2007.

bin/varnishd/mgt/mgt_main.c

@@ -720,9 +720,9 @@ main(int argc, char * const *argv)
 	if (b_arg != NULL && f_arg != NULL)
 		ARGV_ERR("Only one of -b or -f can be specified\n");
 
-	if (S_arg != NULL && *S_arg == '\0') {
+	if (S_arg != NULL && !strcmp(S_arg, "none")) {
 		fprintf(stderr,
-		    "Warning: Empty -S argument, no CLI authentication.\n");
+		    "Warning: CLI authentication disabled.\n");
 	} else if (S_arg != NULL) {
 		VJ_master(JAIL_MASTER_FILE);
 		o = open(S_arg, O_RDONLY, 0);
@@ -822,7 +822,7 @@ main(int argc, char * const *argv)
 	if (d_flag)
 		mgt_cli_setup(0, 1, 1, "debug", cli_stdin_close, NULL);
 
-	if (*S_arg != '\0')
+	if (strcmp(S_arg, "none"))
 		mgt_cli_secret(S_arg);
 
 	if (M_arg != NULL)

doc/sphinx/reference/varnishd.rst

@@ -123,11 +123,11 @@ OPTIONS
   *vmod_path* read only as these can potentially be used to escalate
   privileges from the CLI.
 
--S file
+-S secret-file
 
   Path to a file containing a secret used for authorizing access to
   the management port. If not provided a new secret will be drawn
-  from the system PRNG.
+  from the system PRNG.  To disable authentication use ``none``.
 
 -s <[name=]type[,options]>