8203014: jcmd should output command list if no command is given

Reviewed-by: sspitsyn, simonis

src/jdk.jcmd/share/classes/sun/tools/jcmd/Arguments.java

@@ -88,7 +88,9 @@ class Arguments {
         }
 
         if (listCounters != true && sb.length() == 0) {
-            throw new IllegalArgumentException("No command specified");
+            // Omitting the command shall cause the target VM to print out a list
+            // of available commands.
+            sb.append("help");
         }
 
         command = sb.toString().trim();