Use "none" to disable authentication

This is an adaptation of b2c18706 by Federico G. Schwindt.

Previously you needed an empty argument (''), which didn't play very
well along sysv init scripts and was not documented. Now both "none"
and the empty argument work, but an extra warning is echoed if the
empty argument is used.

The documentation only mentions "none" for disabling the secret, since
this is the only way of doing it in the next version.

Fixes #2007.

bin/varnishd/mgt/mgt_main.c

@@ -717,6 +717,11 @@ main(int argc, char * const *argv)
 	if (S_arg != NULL && *S_arg == '\0') {
 		fprintf(stderr,
 		    "Warning: Empty -S argument, no CLI authentication.\n");
+		fprintf(stderr, "Warning: For forward compatibility,"
+		    " please switch to using -S none\n");
+	} else if (S_arg != NULL && !strcmp(S_arg, "none")) {
+		fprintf(stderr,
+		    "Warning: CLI authentication disabled.\n");
 	} else if (S_arg != NULL) {
 		VJ_master(JAIL_MASTER_FILE);
 		o = open(S_arg, O_RDONLY, 0);
@@ -817,7 +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_dir* 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]>