vncserver: Add -test option to print Xvnc cmd line

This is useful in conjunction with gdb or valgrind.

Closes #448

unix/vncserver.in

@@ -1,7 +1,7 @@
 #!/usr/bin/env perl
 #
-# Copyright (C) 2009-2018, 2020-2022, 2024 D. R. Commander.
-#                                          All Rights Reserved.
+# Copyright (C) 2009-2018, 2020-2022, 2024-2025 D. R. Commander.
+#                                               All Rights Reserved.
 # Copyright (C) 2021 Steffen Kieß
 # Copyright (C) 2010 University Corporation for Atmospheric Research.
 #                    All Rights Reserved.
@@ -213,7 +213,8 @@ foreach $_registrydir (@registrydirs) {
               "-3dwm", 0, "-vgl", 0, "-debug", 0, "-x509cert", 1,
               "-x509key", 1, "-autokill", 0, "-quiet", 0, "-wm", 1,
               "-sessionlist", 0, "-sessionstart", 0, "-novnc", 1,
-              "-noautokill", 0, "-rfbport", 1, "-rfbunixpath", 1, "-uds", 0);
+              "-noautokill", 0, "-rfbport", 1, "-rfbunixpath", 1, "-uds", 0,
+              "-test", 0);
 
 &Usage() if ($opt{'-help'} || $opt{'-h'} || $opt{'--help'});
 
@@ -490,6 +491,10 @@ if (!$opt{'-debug'}) {
 # Run $cmd and record the process ID.
 
 $pidFile = "$vncUserDir/$host:$displayNumber.pid";
+if ($opt{'-test'}) {
+  print "$cmd\n";
+  exit;
+}
 system("$cmd & echo \$! >$pidFile");
 
 # Record the RFB Unix domain socket path